Improve value restriction error message #1103

[Happypig375]

Improve value restriction error message, relates to #1103, fixes #1161

Implemented message for value restriction error

For function values

Value restriction: The value '{0}' has an inferred generic function type
    {1} 
However, values cannot have generic type variables like '_a in "let f: '_a". You should define '{2}' as a function instead by doing one of the following:
- Add an explicit parameter that is applied instead of using a partial application like "let f param"
- Add a unit parameter like "let f()"
- Write explicit type parameters like "let f<'a>",
or if you do not intend for it to be generic, either:
- Add an explicit type annotation like "let f : obj -> obj"
- Apply arguments of non-generic types to the function value in later code for type inference like "do f()".
This error is because a let binding without parameters defines a value, not a function. Values cannot be generic because reading a value is assumed to result in the same everywhere but generic type parameters may invalidate this assumption by enabling type-dependent results.

For non-function values

Value restriction: The value '{0}' has an inferred generic type
    {1}
However, values cannot have generic type variables like '_a in "let x: '_a". You can do one of the following:
- Define it as a simple data term like an integer literal, a string literal or a union case like "let x = 1"
- Add an explicit type annotation like "let x : int"
- Use the value as a non-generic type in later code for type inference like "do x"
or if you still want type-dependent results, you can define '{2}' as a function instead by doing either:
- Add a unit parameter like "let x()"
- Write explicit type parameters like "let x<'a>".
This error is because a let binding without parameters defines a value, not a function. Values cannot be generic because reading a value is assumed to result in the same everywhere but generic type parameters may invalidate this assumption by enabling type-dependent results.

[Happypig375]

Ready for review

[vzarytovskii]

This lgtm, @dotnet/fsharp-team-msft

[psfinaki]

Thank you.

[psfinaki]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[psfinaki]

@Happypig375 looks like this somehow intertwined with the latest parens removal code fix. Would you like to look at it? I can jump on that in the upcoming days if you want to want to bother. Should be something simple though hopefully.

[psfinaki]

Sorry false alarm, updating the branch.

[edgarfgp]

Please do not let this PR stale. the new error message is much better.

[psfinaki]

Thanks for reminding, Edgar.

@Happypig375 will you have a moment to resolve the conflicts and address the remaining notes? If not, I can help with that this week.

[Happypig375]

I can revisit this next week but @psfinaki if you want you can go ahead with this PR

[psfinaki]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[psfinaki]

@Happypig375 so I'll be merging this - thanks for the PR!