Replace `*const` and `*mut` with `&` and `&mut` in mun_abi

[Wodann]

cbindgen supports conversion of references - with a lifetime - into pointers. Currently, we are using raw pointers in our mun_abi, but by converting them to references, we can make our Rust code safer.

• [x] claim this issue (assign yourself or comment below)
• [x] setup repository on your local machine and make sure all tests pass (cargo test)
• [x] read our contributing guidelines
• [x] in mun_abi, replace as many instances of *const and *mut to &'a and &'a mut as possible
• [x] start a Pull Request. Set description to closes #422. If this is your first PR, welcome :tada: :smile:

[Wodann]

The remaining occurrences of raw pointers in the mun_abi are either dynamic arrays or strings. Those cannot currently be converted to safer Rust types:

• cbindgen does not currently support CString or CStr: https://github.com/eqrion/cbindgen/issues/782
• cbindgen also cannot support wide pointers like &dyn Trait or &[T], as their layout and ABI is not guaranteed. In the case of slices you can at least decompose them into a pointer and length, and reconstruct them with slice::from_raw_parts.