[Rust] ABI-compatible string

[awillenbuecher-xq-tec]

We need an ABI-compatible string which behaves similar to Rust's std::string::String, except that it contains a fixed-capacity storage within the data structure itself. The primary use case is for the ABI-Compatible Data Types feature request.

Criteria:

• should share the majority of the logic with the fixed-capacity string implementation
• must never allocate heap memory
• must provide a stable, well-defined memory layout
• operations related to the string's capacity (like push, push_str, and pop) should be non-panicking and return a Result or Option instead