Implement let-syntax and letrec-syntax about husk-scheme HOT 3 CLOSED

Simple versions of let-syntax and letrec-syntax have been implemented, but there are some restrictions. In particular, these constructs do not work correctly if they are used in code that expands as part of a macro. For example: (let () (let-syntax ...

Some high-level design notes on how this could be made to work:

Note from http://www.cs.indiana.edu/~dyb/pubs/LaSC-5-4-pp295-326.pdf:

Also, internal define-syntax forms may appear wherever internal define forms are
permitted, in which case the definitions behave as if introduced by letrec-syntax

So we could transform a letrec-syntax form into another using define-syntax. let-syntax could be handled in the same way, although we would need to walk the macro to ensure that none of the introduced macros reference each other.

from husk-scheme.

Updated the macro walking code to detect the presence of a let*-syntax macro. If one is found it is loaded and recursively expanded in an environment internal to the macro expansion.

Anyway, this passes all of the current tests, but it needs a bit more verification before closing out this ticket.

from husk-scheme.

These forms work well enough that this ticket can be closed out. At this point any issues can be dealt with individually.

from husk-scheme.