Portable rustc Exports

[JustusAdam]

What Changed?

Introduces the rustc_portable module that exports either rustc identifiers (like DefId) or our proxy. Together with cfg_attr this makes it more ergonomic to define data structures that are used outside of rustc but depend on those identifiers.

Why Does It Need To?

I really didn't like the

struct Foo {
    #[cfg(feature = "rustc")]
    #[serde(with = "rustc_proxies::DefId")]
    def_id: def_id:: DefId,
    #[cfg(not(feature = "rustc"))]
    def_id: rustc_proxies::DefId,
}

code blocks. I think they are hard to read and easy to get wrong. Now you can write

struct Foo {
    #[cfg_attr(feature = "rustc", serde(with = "rustc_proxies::DefId"))]
    def_id: rustc_portable:: DefId,
}

Checklist

• Above description has been filled out so that upon quash merge we have a
  good record of what changed.
• New functions, methods, types are documented. Old documentation is updated
  if necessary
• Documentation in Notion has been updated
• Tests for new behaviors are provided
  Works if it compiles 😄
  • New test suites (if any) ave been added to the CI tests (in
    .github/workflows/rust.yml) either as compiler test or integration test.
    Or justification for their omission from CI has been provided in this PR
    description.