Optimizing `call_indirect` performance

Ideas from a brainstorming session with some Cranelift folks:

* [x] Typed funcrefs can statically remove null checks and sig checks; but how do we use?
* In `cranelift-wasm`, need to feed type info from validator to say “non-null”
* [x] Catch calls to null via signal rather than explicit check
* [ ] Inline `VMFuncRef`s into table, instead of having table be a vec of pointers to `VMFuncRef`s
* [ ] Store defined tables directly in the vmctx, rather than indirectly
* Only works for max size tables
* Helps with allocating multiple tables inside pooling allocator
* [x] If min and max are equal then table size is constant, don’t load table size from memory, just use the constant
* [x] If table index is constant and less than minimum, skip bounds check

And, of course, it would be nice to have some (micro) benchmarks to measure all of this.