Adds a new `fixtures` crate intended for running end-to-end tests of the
compiler. As we expand the compiler this will eventually match our JS fixture
setup, where we have .js files as input and produce memoized JS output.
For now, this does the following:
* Parses with SWC (omg this was painful to setup)
* Runs SWC's name resolution, which annotates the SWC ast in-place
* Convert the SWC ast into our `estree` representation
* Convert `estree` into `hir` for each top-level function declaration in the
input program
* Print the Rust `Debug` view of the resulting HIR
As a next step i'll add a pretty-printer for the HIR to roughly match what we
have in JS.