[Experimental simulators] Improve C API exposed by qdk_sim_rs.

[cgranade]

As suggested by @kuzminrobin, there's several opportunities available to improve c_api.rs to better align with C API design guidelines and to offer a more robust interface from C callers into the Rust library:

• Add new deallocate_string function. To ensure that strings are allocated and deallocated in the same binary, C callers should be able to return ownership of strings back to Rust for deallocation.
• Change usize to u64. Since usize is architecture-dependent, changing to u64 is more portable.
• Granular return codes. Return codes from C API functions could be differentiated (as opposed to all -1) to allow for more granular reporting of errors to C-language callers.