move lucet-validate into lucetc crate

[pchickey]

lucet-validate was creating more trouble as a separate crate than it was worth. And, to fix some design problems, I ended up rewriting it piece-by-piece.

• lucet-validate crate is gone. What is left of it and its tests are merged into lucetc. This eliminates a dev-dependencies loop that caused us problems with cargo publish.
• lucetc::validate now uses a builder interface for construction, which is more flexible than before. It supports both wasi commands and reactors, though we don't do anything with reactors at the moment (lucet-runtime doesnt know what to do with them yet).
• a Validator, once constructed, gets its register_import and register_export methods called by the ModuleInfo struct's ModuleEnvironment impl. This means that as cranelift_wasm::translate_module parses the wasm module, we are validating each function import and export. Previously, lucet-validate had a ModuleType struct that was constructed by a totally separate parsing pass on the wasm module; this was never a great design, it was just the quick and dirty version.
• Rather than fail on the first error, a Validator collects all of its errors internally. After translate_module is complete, we get the whole set of validation errors out of Validator and, if there are any, report them all and stop compilation.
• Tests were added for the new behavior.

The PR is arranged as a series of commits that transform the lucet-validate code into the final form step by step, with tests passing at each commit, if you want to read it that way.

This PR closes #438: now all validation errors are reported, with positive and negative tests.

[cratelyn]

Great work! Thank you 🙂

[cratelyn]

♻️ ✔️