Merge #1267

1267: Macro expand to r=edwin0cheng a=matklad

closes #1264 

The core problem this PR is trying to wrangle is that macros can expand to different stuffs, depending on context. 

That is, `foo!()` on the top-level expands to a list of items, but the same `foo!()` in expression position expands to expression.

Our current `hir_parse(HirFileId) -> TreeArc<SourceFile>` does not really support this. 

So, the plan is to change `hir_parse` to untyped inreface (`TreeArc<Syntaxnode>`), and add `expands_to` field to `MacroCallLoc`, such that the *target* of macro expansion is selected by the calling code and is part of macro id. 

This unfortunately looses some type-safety :( 


Moreover, this doesn't really fix #1264 by itself, because we die due to some other error inside macro expansion: expander fails to produce a tree with a single root, which trips assert inside rowan.

Co-authored-by: Aleksey Kladov <[email protected]>

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