Turn the `wasmtime-runtime` crate into the `wasmtime::runtime::vm` module

[fitzgen]

We want to merge them together because there is no abstraction or layering or boundary between them in practice, and our architectures and designs have, at times, been constrained by this artificial crate boundary and what is visible in which crate and this has forced us to use nasty workarounds like defining traits in wasmtime-runtime that get implemented in wasmtime and used as dynamic trait objects inside wasmtime-runtime. Merging them will allow us to remove these hacks.

[github-actions[bot]]

Label Messager: wasmtime:config

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[fitzgen]

The next step after this, before actually merging the crates together, is to factor out wasmtime_runtime::{sys,mmap,mmap_vec} into its own crate, since this functionality is used directly by the CLI in one place, and I don't want to add that stuff to the wasmtime crate's public API.

This is proving to be a bit hairier than anticipated; investigating alternatives. Shouldn't affect this PR.