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.

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