Wasm test utilities

This crate contains:

• several small crates that compile to Wasm and are used as test values.
• enum TestWasm which enumerates all of those crates.
• impl From<TestWasm> for DnaWasm to obtain the compiled Wasm artifacts for those crates.
• a build.rs file that builds all those crates for compile-time inclusion in the library.

These Wasm crates directly test the host/guest implementation of Holochain without going through an HDK or other convenience interface.

We do this to make sure that it stays reasonably easy to interact with Holochain without using the hdk and holochain_wasmer_* crates.

The tests that run this Wasm generally sit in the ribosome.rs module in core. This is necessary because the Wasm crates depend on certain global functions that core defines and needs to inject.