Reduce `wasmi::Engine` memory consumption for storing function artifacts about wasmi

[Robbepop]

Depends on new wasmparser updates:

• bytecodealliance/wasm-tools#1495 (mandatory)
• bytecodealliance/wasm-tools#1496 (optional)

[Robbepop]

Using the following definitions for Wasmi's code_map.rs:

pub struct UncompiledFuncEntity {
    func_index: u32,
    bytes: SmallByteSlice,
    module: ModuleHeader,
    validate: Option<wasmparser::ValidatorResources>,
}

pub enum SmallByteSlice {
    Small {
        len: u8,
        bytes: [u8; 22],
    },
    Big(Box<[u8]>),
}

• SmallByteSlice now stores 22 bytes inline instead of 30.
  • Note: With union and unsafe Rust trickery we could decrease the size of this type by another 8 bytes, however, for now I don't think the gains are worth this refactor.
• Replaced func_to_validate: Option<wasmparser::FuncToValidate<wasmparser::ValidatorResources>> field with validate: Option<wasmparser::ValidatorResources> which is a lot smaller. The index and ty fields are redundantly stored in UncompiledFuncEntity::func_index field with access to module if we do some refactoring in ModuleHeader. Furthermore the features: WasmFeatures field is the same for all uncompiled function entities in the same Engine and thus can be stored in the CodeMap directly and reused.

All in all this nets us the following improvements in memory consumption:

• size_of<UncompiledFuncEntity>(): 88 -> 48
• size_of<InternalFuncEntity>(): 88 -> 48

Therefore, 40 bytes savings per function processed by the Engine.
For 1000 functions this is equivalent to 40kB.

[Robbepop]

Blocked by bytecodealliance/wasm-tools#1493 and bytecodealliance/wasmtime#8341 (comment).