Add %%local-alias

[jpellegrini]

@egallesio this is a work in progress. I am including it here just so you can see it, but I suppose its not yet time to accept it.

This is the idea I was talking about in issue #55 .
It was actually not complex at all. With %%local-alias, it would be possible to implement hygienic macros in a clean and not complex way.

(%%local-alias b a) defines b as an alias to the a variable visible where the %%local-alias is used. It does so by pushing one more item onto the env stack: instead of a variable name (symbol), it pushes a list (the alias). The rest of the code is adapted to cope with this.

[jpellegrini]

For full compliance with the standards, it would also be necessary to have a similar %syntax-alias.

> (define-syntax let (syntax-rules () ((let a b c) (+ a b c))))
> (let 2 3 4)
9

(All the issues with variables arise with syntax too, but the solution then is very similar. Working on it)

[jpellegrini]

This PR needs a complete rewrite -- I'll do that later when we have lexically-scoped define-macro

[lassik]

Please note that SRFI 212: Aliases is currently in draft status. Possibly related, or STklos might have some ideas that are good to add to that SRFI.

[jpellegrini]

Please note that SRFI 212: Aliases is currently in draft status. Possibly related, or STklos might have some ideas that are good to add to that SRFI.

Yes - but it would make more sense to implement that after the work on lexically-scoped macros is done, because both variables and macros could have their names aliased (indeed, my idea was to use both in order to implement R7RS semantics for syntax-rules).