Simplified generator system (#657)

## Changes

* This is a complete rewrite of the generator system.
* Old `GenerateMultihistory` syntax is removed.
* Instead, all generators have the format

  ```wl
  Generator[System[rules], param1 -> value1, ...] @ init
  ```

  Note that `param1`, etc. are symbols rather than strings. They have usage messages, etc. Also, one can use lists or associations instead, e.g.,

  ```wl
  Generator[System[rules], {param1 -> value1}, <|param2 -> value2|>, param3 -> value3] @ init
  ```

* Parameters are now declared separately from systems and generators.
* Systems need to declare a logical expression specifying which parameters can be specified. For example,

  ```wl
  Implies[MaxEvents || MaxDestroyerEvents, EventOrder]
  ```

  means that `EventOrder` needs to be specified if either `MaxEvents` or `MaxDestroyerEvents` are specified.

* Generators have predefined values for some parameters. E.g., `GenerateSingleHistory` sets `MaxDestroyerEvents -> 1`, which can no longer be changed.

## Comments

* Apologies for a huge PR. There is a lot of refactoring here as all instances of `GenerateMultihistory` had to be changed.
* Ordering functions page is deleted for now but should return as a page for the `EventOrder` parameter once it is used somewhere.
* @daneelsan, unfortunately, it will break #643, but on the flip side, it should make it a lot easier to define parameters (as one does not need to think where to put them anymore).

## Examples

* `GenerateSingleHistory`:

```wl
In[] := #["StatesList"] & @
 SetReplaceTypeConvert[{WolframModelEvolutionObject, 2}] @
  GenerateSingleHistory[MultisetSubstitutionSystem[{a_, b_} :> {a + b}], MaxEvents -> 4] @ {1, 2, 3}
Out[] = {{1, 2, 3}, {3, 3}, {6}}
```

* `GenerateAllHistories`:

```wl
In[] := #["ExpressionsEventsGraph", VertexLabels -> Placed[Automatic, After]] & @
 SetReplaceTypeConvert[{WolframModelEvolutionObject, 2}] @
  GenerateMultihistory[
    MultisetSubstitutionSystem[{a_, b_} /; a < b :> {a + b}], MaxGeneration -> 2] @ Range[3]
```
<img width="399" alt="image" src="https://user-images.githubusercontent.com/1479325/122285946-f08ade80-ceb4-11eb-882d-7da3fdeb75fb.png">

Documentation/Generators/$SetReplaceGenerators.md

@@ -0,0 +1,9 @@
+# $SetReplaceGenerators
+
+**`$SetReplaceGenerators`** gives the list of all generators that can be used to evaluate
+[computational systems](/Documentation/Systems/README.md):
+
+```wl
+In[] := $SetReplaceGenerators
+Out[] = {GenerateMultihistory, GenerateSingleHistory}
+```

Documentation/Generators/$SetReplaceSystems.md

@@ -1,7 +1,7 @@
 # $SetReplaceSystems
 
 **`$SetReplaceSystems`** gives the list of all [computational systems](/Documentation/Systems/README.md) that can be
-used with [GenerateMultihistory](/Documentation/Generators/GenerateMultihistory.md) and related functions:
+used with [GenerateMultihistory](/Documentation/Generators/GenerateMultihistory.md) and other [generators](README.md):
 
 ```wl
 In[] := $SetReplaceSystems

Documentation/Generators/GenerateMultihistory.md

@@ -1,24 +1,15 @@
 # GenerateMultihistory
 
-**`GenerateMultihistory`** is the most verbose and configurable generator. It can create both single histories, full
-multihistories, as well as partial ones. It, however, requires one to specify all groups of parameters
-([system](/Documentation/Systems/README.md), [event selection](EventSelectionParameters.md), deduplication,
-[event ordering](EventOrderingFunctions.md) and [stopping conditions](StoppingConditionParameters.md)), nothing is
-implied automatically:
-
-```wl
-GenerateMultihistory[
-  system, eventSelectionSpec, tokenDeduplicationSpec, eventOrderingSpec, stoppingConditionSpec] @ init
-```
+**`GenerateMultihistory`** is the most configurable multihistory generator. With no parameters specified, it attempts to
+generate all possible histories starting from the initial state. However, it can be configured to generate partial
+multihistories and even single histories, although [`GenerateSingleHistory`](GenerateSingleHistory.md) is more
+convenient for that.
 
 For example, for a [`MultisetSubstitutionSystem`](/Documentation/Systems/MultisetSubstitutionSystem.md):
 
 ```wl
-In[] := multihistory = GenerateMultihistory[MultisetSubstitutionSystem[{a_, b_} :> {a + b}],
-                                            {"MaxDestroyerEvents" -> 3},
-                                            None,
-                                            EventOrderingFunctions[MultisetSubstitutionSystem],
-                                            {"MaxEvents" -> 10}] @ {1, 2, 3, 4}
+In[] := multihistory = GenerateMultihistory[
+  MultisetSubstitutionSystem[{a_, b_} :> {a + b}], MaxDestroyerEvents -> 3, MaxEvents -> 10] @ {1, 2, 3, 4}
 ```
 
 <img src="/Documentation/Images/MultisetMultihistory.png" width="472.2">
@@ -29,11 +20,3 @@ In[] := #["ExpressionsEventsGraph", VertexLabels -> Placed[Automatic, After]] &
 ```
 
 <img src="/Documentation/Images/GenerateMultihistoryExample.png" width="478.2">
-
-Everything that can be generated with more specialized `GenerateSingleHistory` and `GenerateFullMultihistory` can be
-reproduced with `GenerateMultihistory` as well. This can be done by setting `"MaxDestroyerEvents" -> 1` to emulate
-`GenerateSingleHistory` and setting `"MaxDestroyerEvents" -> Infinity` and leaving the
-[stopping condition](StoppingConditionParameters.md) empty to emulate `GenerateFullMultihistory`.
-
-However, by setting `"MaxDestroyerEvents"` to finite values larger than 1, one can generate multihistories not possible
-with other generators.