[DIP 1007] Allow to use keywords as identifiers with a new special token sequence

[ghost]

This PR is the implementation of the possible DIP 1007 (warning, not official, the document is not merged)

It shows how it would be simple to use a lexical rule that allows to use any keyword as identifier, without breaking changes. This is based on the usage of # which is already used for special token sequences.
The new special token sequence is simply # Keyword. Read the DIP for more detailed information.

[PetarKirov]

BTW, the same is accomplished in C# with string @for (i.e. prefixing the keyword with @).

https://msdn.microsoft.com/en-us/library/x53a06bb.aspx

https://msdn.microsoft.com/en-us/library/8d7y7wz6.aspx

Keywords are predefined, reserved identifiers that have special meanings to the compiler. They cannot be used as identifiers in your program unless they include @ as a prefix. For example, @if is a valid identifier but if is not because if is a keyword.

As a former C# user, in my experience, this feature is occasionally useful and has never been a problem of any sort in practice, so I think this PR would be a nice addition to the language. Personally I would prefer a different syntax, than #keyword, but we can discuss this at later stage.

[ghost]

# is the only available ascii token that has no impact on the complexity. For the others you have to make a forward lookup each time it happens even if you know it's unlikely to be a keyword escape case.
# is rare, and that's why it's good. You say yourself that

this feature is occasionally useful