`wasm-smith`: add support for generating instructions from the `threads` proposal

[fitzgen]

Right now, wasm_smith::Config::threads_enabled only configures whether we will generate shared memories or not. We should also emit atomic instructions and wait/notify instructions.

The instructions we should support: https://webassembly.github.io/threads/core/syntax/instructions.html#syntax-instr-atomic-memory

The place we need to add that support:

wasm-tools/crates/wasm-smith/src/core/code_builder.rs

Lines 76 to 93 in 44b5ea4

	// The static set of options of instruction to generate that could be valid at
	// some given time. One entry per Wasm instruction.
	//
	// Each entry is made up of up to three parts:
	//
	// 1. A predicate for whether this is a valid choice, if any. `None` means that
	// the choice is always applicable.
	//
	// 2. The function to generate the instruction, given that we've made this
	// choice.
	//
	// 3. The `InstructionKind` the instruction belongs to; this allows filtering
	// out instructions by category.
	//
	// 4. An optional number used to weight how often this instruction is chosen.
	// Higher numbers are less likely to be chosen, and number specified must be
	// less than 1000.
	instructions! {

[danielvallance]

Hello @fitzgen , I have not worked on this repo before however I would like to get involved and so I have created a pull request for this issue: #2386

If you would like me to change anything please let me know, thanks!

[abrown]

This was completed in #2386; eventually we should add the shared-everything-threads instructions as well!