Add a new crate to generate host WIT code

[jvff]

Motivation

wit-bindgen has recently been changed to only generate guest-side code. Host-side code must now be generated by separate crates specific for each Wasm runtime. Also, the generated host code didn't support reentrant calls.

Solution

Create a new crate to generate the host side code for both the Wasmer and Wasmtime runtimes and that supports reentrant calls.

The new crate should also use Rust code as the source-of-truth, and support generating WIT files for wit-bindgen to use to generate the guest code.

Merge plan

Since this new crate is non-trivial, merging it will be split into several PRs.

• Create Witty crate (Create initial Witty crate with primitive type definitions #880)
• Represent WIT memory layouts (Add memory layout traits to Witty #886)
• Create WitType trait (Create the WitType trait #890)
• Add runtime and memory traits (Add abstraction to access guest Wasm module memory #898)
• Create WitLoad trait (Create a WitLoad trait #908)
• Create WitStore trait (Create a WitStore trait #915)
• Add support for enums (Add support for WIT variants #921)
• Create a wit_import macro (Add wit_import attribute macro to import functions from Wasm modules #944)
• Add support for Wasmer (Add support for Wasmer in Witty #945)
• Add support for Wasmtime (Add support for Wasmtime in Witty #947)
• Create a wit_export macro (Add wit_export attribute macro to export host functions to Wasm modules #955)
• Add support for reentrancy (Add reentrancy support to Witty #960)
• Add support for extra user data (Support custom user data in host functions exported with Witty #1310)
• Generate WIT files from Rust code
  • Generate WIT type declarations #1704
  • Create a WitInterface trait to allow creating of WIT interface snippets #1902
  • Add WIT file contents generation to Witty #1910