Investigate adding a "fast init" function for initializing the `VMContext`

[alexcrichton]

In #13487 support was added to initialize a wasm instance's "complicated" globals, tables, memories, etc. This, in theory, can go even further, however, and we could JIT initialization of an instance's VMContext itself. Right now vm::Instance::initialize_vmctx is a very branch-y and complicated function. This low-level initialization additionally doesn't need anything like fuel checks, epochs, a wasm stack, etc. Ideally we could split module initialization in two:

• A "fast" function which has a custom native ABI generated by Cranelift which initializes the VMContext. The signature of this function need not be related to wasm (could take native pointers) and need not be connected to things like VMFuncRef for example. This could largely just be a function pointer emitted by Cranelift which is virtually dispatched to during instantiation
• A "slow" function which is implemented in Move most module initialization to compiled code  #13487 for full wasm initialization which needs fuel, epochs, a custom stack, etc. This function could trap (unlike the "fast" function), and is that the "module startup" is after Move most module initialization to compiled code  #13487.

This in theory might make module instantiation even faster after #13487 since initialize_vmctx would be a JIT'd function rather than a generic one-size-fits-all function with lots of branches. Benchmarking TBD!