Add --disable-parallel-compilation CLI flag

[TerrorJack]

Closes #4910. This PR adds --disable-parallel-compilation CLI flag to wasmtime, for use cases when parallel compilation is not desirable.

[alexcrichton]

Thanks for this!

Parallel is a compile-time-conditional feature as well which is why the CI is failing here since the parallel compliation in the cli-flags crate is disabled when the crate is built in isolation. The feature is enabled by default in wasmtime and wasmtime-cli, however.

To fix the issue you'll want to add a parallel-compilation flag to the cli-flags crate and additionally enable the feature in the wasmtime-cli crate when the wasmtime-cli crate's parallel-compilation feature is enabled.

[TerrorJack]

@alexcrichton Thanks for the suggestion! CI now passes, given my added code in cli-flags crate touches the parallel_compilation config field anyway, I've simply added wasmtime/parallel-compilation to cli-flag crate's default features.