Generalize and make less brittle

[Shfty]

• Introspecting the structure of PropertyIdentifier / Identifier and relying on implementation details of its ToTokens impl in order to sub in passthrough identifiers is not ideal

• Need to be able to inject arbitrary literals from the outer rust context without specializing too hard around structure

  • Baseline is arbitrary types of identifiers, but general IR substitution would be ideal
  • i.e. Being able to bind an elysianexpr! result, and inject it directly into another elysian*! invocation without needing to inline

• Should be able to inject rust attributes to mark elements for replacement instead of checking against specifically-named idents

  • How to preserve these attributes across the Rust -> IR -> Rust boundary?
  • May need to integrate more tightly so it skips concrete IR and goes straight from Rust -> Rust
  • Some sort of ToTokens wrapper trait that can introduce metadata via param?

[Shfty]

DONE: Reimplemented using a Rust -> Rust parser that interpolates by default, and uses a # prefix to compose pre-converted IR