WebAssembly backend

[womeier]

This PR adds our Wasm backend, as dicussed with Zoe.
It's not ready to be merged. (The proofs need some cleanup, see also below.)

However, you could already take a look at the main statement.
We extended the plugin with CertiCoq Compile Wasm <definition>.. Benchmarks are in benchmarks/wasm.

We'd like to do at least the following before merging:

• depend on upstream WasmCert instead of our fork
• use Disjoint and bound_var instead of NoDup (for end-to-end statement)
• general cleanup of the proofs (we'll probably do that in January 25)
• these helper lemmas are copied from the C backend and belong somewhere else I think
• don't depend on the coqprime package (we use it for testing only, is there a way to specify development dependencies for opam?)

This PR has our backend in a single commit, we'll keep the full history at gh/womeier/certicoqwasm.