transpile: Add AST node parent information

[Rua]

• Depends on transpile: Split off various c_ast submodules #1652.

This collects information about the parent nodes of each AST node (except types). It's not currently used for anything, but could potentially be used to solve #1321, by allowing conversion functions across the transpiler to easily find the types of their parent nodes.

[ahomescu]

I don't think we should merge a massive change that isn't used for anything. Edit: unless we also want #1679, in which case this is a prerequisite and not an independent change.

[Rua]

There are also other things that this makes possible, not all of which have been considered yet necessarily. One thing that comes to mind is being able to completely replace ExprContext, so that all context information is available globally and no longer relies on passing things down the call stack. That may be a requirement for making a fix for #1321, since macro translation is done outside the call stack and thus doesn't currently know what context each of its expansions exists in.

EDIT: To illustrate, the ctx being passed here is almost certainly wrong, because it's the context of the current expansion site, and not of the MacroObject itself, nor of all expansion sites:

c2rust/c2rust-transpile/src/translator/mod.rs

Line 4179 in c11f9aa

self.convert_decl(ctx, *macro_id)?;