Memory checker (wmemcheck) completeness

This issue is meant to track what remains to make `wmemcheck` feature-complete and robust enough for practical use:

- [ ] Handle SIMD loads and stores (#6829)
- [ ] Hook `calloc` as well as existing `malloc` and `free`
- [ ] Provide hooks for custom allocators to use
- [ ] Allow for more flexibility in heap and stack layout (don't assume stack is above `.data`/`.bss` and everything below initial TOS is accessible)
- [ ] Provide an alternate way to hook `malloc`/`free`, rather than requiring a name section in the `.wasm` module
- [ ] Provide an option to print warnings as violations occur, rather than halting execution (like Valgrind)
- [ ] Provide a mechanism to define "suppressions" for unwanted warnings (like Valgrind)
- [ ] Track definedness at byte or bit level?

cc @iximeow for more thoughts, and others from today's Wasmtime meeting?