get rid of parity-wasm

[hawkw]

it would be nice to replace the dependency on (our fork of) parity-wasm with a dependency on wasmtime.

wasmtime would be preferable for the following reasons:

• it supports executing wasms in an async context (relative to the host), with async host calls. this is vital to allow tasks executing wasms to yield to the mycelium runtime.
• it's almost certainly a higher quality implementation than the parody one
• some of my friends work on it

migrating to wasmtime would require adding #![no_std] support to wasmtime (and presumably some of its dependencies). this could be started in a fork, but it would be cool to upstream it eventually if the maintainers are interested...

[hawkw]

@cratelyn at one point i think you mentioned being interested in helping out with some of this?