This allows macros to take a keyword dict containing useful things by
defining a keyword argument. This allows us to pass in new objects
which might be handy to have in macros.
This changeset refactors module_name to become `compiler`, so that we
can pass the compiler itself into the macros as `opts['compiler']`.
This allows the macro to both get the macro name
(`compiler.module_name`), as well as use the compiler to build AST.
In the future, this will enable us to create "super-macros" which return
AST, not HyAST, in order to manually create insane things from userland.
For userland macros (not `defmacro`) the core.language `macroexpand`
will go ahead and make a new compiler for you.
A macro is available in the module where it was defined and
in any module that does a require of the defining module.
Only macros defined in hy.core are globally available.
Fixes#181
