Code quality issue: inconsistent Ruby to Rust string conversion

[nickysn]

This is a refactor task.

The code in gems/codetracer-ruby-recorder/ext/native_tracer/src/lib.rs contains several similar repeated patterns of code for converting from Ruby strings to Rust strings. To reveal them, search for RSTRING_PTR in the code. These patters should probably be unified and extracted to a function or two. However, they differ in various corner cases, so we should carefully decide which version to use:

1. Some convert NIL values to "", some don't bother with this check.
2. Some use String::from_utf8_lossy, others use std::str::from_utf8. These vary in the way they handle invalid UTF-8 data.
3. Some convert non-string types to string, others don't.

[nickysn]

After a certain amount of work, I've extracted the commonly repeated code to 5 functions, representing different variations of the code. Furthermore, by removing value_to_string and replacing it with value_to_string_exception_safe, I've further reduced the string conversions to only these 4:

rstring_lossy(val: VALUE) -> String
rstring_checked(val: VALUE) -> Result<String, FromUtf8Error>
rstring_checked_or_empty(val: VALUE) -> String
value_to_string_exception_safe(recorder: &Recorder, val: VALUE) -> String