Fix macros producing Expr(:toplevel)

[mlechu]

Addresses #39 on the Expr side with a slightly simpler implementation than before. We may need to store more information in the hygienic_scope node on the SyntaxTree side. This change is mainly to get @enum to work.

I've put an explanation of the behaviour in the docstring:

"""
Insert a hygienic-scope around each arg of K"toplevel" returned from a macro.

It isn't correct for macro expansion to recurse into a K"toplevel" expression
since one child may define a macro and the next may use it.  However, not
recursing now means we lose some important context: the module of the macro we
just expanded, which is necessary for resolving the identifiers in the
K"toplevel" AST.  The solution implemented in JuliaLang/julia#53515 was to save
our place and expand later using `Expr(:hygienic-scope toplevel_child mod)`.

Of course, these hygienic-scopes are also necessary because existing user code
contains the corresponding escaping, which would otherwise cause errors. We
already consumed the hygienic-scope that comes with every expansion, but won't
be looking for escapes under :toplevel, so push hygienic-scope under toplevel
"""

[c42f]

I don't think this works for multiply nested esc() inside the toplevel expression? Or for any occurrence of identifiers inside the top level expression with scope_layer attached? Ie, not a general solution but these cases were already broken?

It would be nice to make @enum work so I think it's fine to merge with some comment (or @test_broken) about the cases which are still broken.

[mlechu]

We may not need to deal with those cases, though I'll add a comment because I'm not certain.

• Do we need to handle scope_layer when locals can't cross the :toplevel boundary?
• I think double-escaping within :toplevel would have the same problems it has with flisp lowering. The only way to escape multiple times in a macro and have it work is to either be introducing the hygienic scopes you're escaping from, or be in control of all the macros that call you. In either case, you're in control of all hygienic-scopes around the :toplevel and double-escaping within it can't touch those.

[mlechu]

Oops, forgot to change the base branch. I'll just wait for docsystem-edge-cases to be merged.