Bindings IR

Adding a new system for bindings generation, modeled after a compiler
pipeline with a lot of small steps.  See the internals docs for a
high-level description of this.

This commit only contains the general pipeline, not any
language-specific ones.  The plan is to use this to create a 
`uniffi-bindgen-gecko-js` pipeline, then start switching over
Python/Swift/Kotlin.

Author: bendk

.cargo/config.toml

@@ -11,6 +11,7 @@ release-backend-crates = [
     "-p", "uniffi_internal_macros",
     "-p", "uniffi_macros",
     "-p", "uniffi_meta",
+    "-p", "uniffi_pipeline",
     "-p", "uniffi_testing",
     "-p", "uniffi_udl",
 ]