Delete most of abi from wit-parser (bytecodealliance#1149)

This commit deletes most of the `abi` module from the `wit-parser`
crate. This functionality is entirely untested in this repository and
was originally intended for `wit-bindgen`-style generators. In thinking
about bytecodealliance/wit-bindgen#622 I'm going to want to make some
invasive changes to this and I'd rather develop everything close to the
source rather than across two repositories.

My plan is to move all this code upstream into the `wit-bindgen-core`
crate in the `wit-bindgen` repository. Which is to say this is the first
half of moving the code, deleting the source here, and I'll make a
follow-up PR to re-add all this code into `wit-bindgen-core` so all
current consumers will continue to work and/or can switch to depending
on `wit-bindgen-core` to preserve the functionality.