Improve SCIP symbols

[mgsloan]

In particular, the symbol generation before this change creates a lot of symbols with the same name for different definitions. This change makes progress on symbol uniqueness, but does not fix a couple cases where it was unclear to me how to fix (see TODOs in scip.rs)

Before this change SymbolInformation provided by a document was the
info for all encountered symbols that have not yet been emitted. So,
the symbol information on a Document was a mishmash of symbols
defined in the documents, symbols from other documents, and external
symbols.

After this change, the SymbolInformation on documents is just the
locals and defined symbols from the document. All symbols referenced
and not from emitted documents are included in external_symbols.

Other behavior changes:

• scip command now reports symbol information omitted due to symbol collisions. Iterating with this on a large codebase (Zed!) resulted in the other improvements in this change.

• Generally fixes providing the path to nested definitions in symbols. Instead of having special cases for a couple limited cases of nesting, implements Definition::enclosing_definition and uses this to walk definitions.

• Parameter variables are now treated like locals.

  • This fixes a bug where closure captures also received symbols scoped to the containing function. To bring back parameter symbols I would want a way to filter these out, since they can cause symbol collisions.

  • Having symbols for them seems to be intentional in 27e2eea, but no particular use is specified there. For the typical indexing purposes of SCIP I don't see why parameter symbols are useful or sensible, as function parameters are not referencable by anything but position. I can imagine they might be useful in representing diagnostics or something.

• Inherent impls are now represented as an impl type which takes the self type as a type parameter.

• Trait impls are now represented as an impl type which takes the self: trait type constraint as a parameter.

• Associated types in traits and impls are now treated like types instead of type parameters, and so are now suffixed with # instead of wrapped with []. Treating them as type parameters seems to have been intentional in 73d9c77 but it doesn't make sense to me, so changing it.

• Static variables are now treated as terms instead of Meta, and so receive . suffix instead of :.

• Attributes are now treated as Meta instead of Macro, and so receive : suffix instead of !.

• enclosing_symbol is now provided for labels and generic params, which are local symbols.

• Fixes a bug where presence of ' causes a descriptor name to get double wrapped in backticks, since both fn new_descriptor and scip::symbol::format_symbol have logic for wrapping in backticks. Solution is to simply delete the redundant logic.