v0 mangling scheme

[tamaroning]

Related to #305

Add support for v0 mangling scheme.

• -C symbol-mangling-version=[legacy|v0] equivalent option
  • -frust-mangling=[legacy|v0]
• identifiers
  • disambiguator
• crates with disambiguation
  • Initial implementation of v0 mangling #2615
• (generic) functions
  • Initial implementation of v0 mangling #2615
• modules, constants, statics
  • Initial implementation of v0 mangling #2615
• inherent impls
  • path of impl-path
  • disambiguator of impl-path
  • Initial implementation of v0 mangling #2615
• trait impls
  • path of impl-path
  • disambiguator of impl-path
  • Initial implementation of v0 mangling #2615
• generic ADTs (e.g. struct <T> S)
  • require refactoring to CanonicalPath and the resolver
• closures
  • Using HIR ID as disambiguator (correct?)
  • v0-mangle closures #2722
• backref (for efficiency)

Refs

• https://rust-lang.github.io/rfcs/2603-rust-symbol-name-mangling-v0.html