While these classes are used by the codegen implementation, conceptually
they're part of `libcst.metadata`, so we should export them from
`libcst.metadata` instead of the top-level `libcst` package.
This introduces a few new gotchas (namely attribute access and a bug with
Union[Callable]), but it also removes a whole host of pyre-fixmes and gets us
updated to a release of pyre that came out after May.
This was a pain, because apparently we can't refer to
"static_analysis.libcst" as a module before its finished executing, due
to https://bugs.python.org/issue25294. To combat this, I had to change
to direct importing inside parser, which was a bit of a pain due to the
number of nodes used. But, it works.
