Clarify execution section. (graphql#221)

* Clarify execution section.

This adds algorithms to the section on execution and reorders content to better follow the flow of execution.

Note that no additional semantics are being introduced in this PR. This is simply algorithmic clarification of the execution process.

* Follow up improvements thanks to @jjergus feedback

* Another pass at further improvements to describing these operations. Included @jjergus's suggestion of getting rid of the tuple based response keying.

* Note about the purpose of initial value

* Add default value rules, further error throwing spots

spec/Section 3 -- Type System.md

@@ -775,15 +775,50 @@ An input object is never a valid result.
 
 **Input Coercion**
 
-The input to an input object should be an unordered map, otherwise an error
-should be thrown. The result of the coercion is an unordered map, with an
-entry for each input field, whose key is the name of the input field.
-The value of an entry in the coerced map is the result of input coercing the
-value of the entry in the input with the same key; if the input does not have a
-corresponding entry, the value is the result of coercing null. The input
-coercion above should be performed according to the input coercion rules of the
+The value for an input object should be an input object literal or an unordered
+map, otherwise an error should be thrown. This unordered map should not contain
+any entries with names not defined by a field of this input object type,
+otherwise an error should be thrown.
+
+If any non-nullable fields defined by the input object do not have corresponding
+entries in the original value, were provided a variable for which a value was
+not provided, or for which the value {null} was provided, an error should
+be thrown.
+
+The result of coercion is an environment-specific unordered map defining slots
+for each field both defined by the input object type and provided by the
+original value.
+
+For each field of the input object type, if the original value has an entry with
+the same name, and the value at that entry is a literal value or a variable
+which was provided a runtime value, an entry is added to the result with the
+name of the field.
+
+The value of that entry in the result is the outcome of input coercing the
+original entry value according to the input coercion rules of the
 type declared by the input field.
 
+Following are examples of Input Object coercion for the type:
+
+```graphql
+input ExampleInputObject {
+  a: String
+  b: Int!
+}
+```
+
+Original Value          | Variables       | Coerced Value
+-------------------------------------------------------------------------------
+`{ a: "abc", b: 123 }`  | `{}`            | `{ a: "abc", b: 123 }`
+`{ a: 123, b: "123" }`  | `{}`            | `{ a: "123", b: 123 }`
+`{ a: "abc" }`          | `{}`            | Error: Missing required field {b}
+`{ b: $var }`           | `{ var: 123 }`  | `{ b: 123 }`
+`{ b: $var }`           | `{ var: null }` | Error: {b} must be non-null.
+`{ b: $var }`           | `{}`            | Error: {b} must be non-null.
+`{ b: $var }`           | `{}`            | Error: {b} must be non-null.
+`{ a: $var, b: 1 }`     | `{ var: null }` | `{ a: null, b: 1 }`
+`{ a: $var, b: 1 }`     | `{}`            | `{ b: 1 }`
+
 #### Input Object type validation
 
 1. An Input Object type must define one or more fields.