Improve performance of get_string

[olivergillespie]

Overview

Speeds up get_string in two ways:

1. Cache the java/lang/String class instead of calling FindClass repeatedly.
2. Use IsInstanceOf instead of GetObjectClass + IsAssignableFrom. java/lang/String is final, so these are equivalent.

Running the benchmark on my machine, change 1:

jni_get_string          time:   [445.86 ns 448.89 ns 454.23 ns]
                        change: [-53.617% -45.104% -39.388%] (p = 0.00 < 0.05)
                        Performance has improved.

Changes 1+2:

jni_get_string          time:   [406.30 ns 406.37 ns 406.45 ns]
                        change: [-9.6565% -9.1695% -8.8758%] (p = 0.00 < 0.05)
                        Performance has improved.

Fixed #530. Also incidentally fixes #528.

Questions/notes

• I don't write Rust, so sorry if I made any basic errors.
• I tried to use std::OnceLock, but get_or_try_init is not yet stable, and I couldn't find a neat way to handle errors, hence pulling in once_cell. Open to ideas here.
• Let me know your thoughts on the changelog and doc changes.

Thanks!

Definition of Done

• There are no TODOs left in the code
• Change is covered by automated tests
• The coding guidelines are followed
• Public API has documentation
• User-visible changes are mentioned in the Changelog
• The continuous integration build passes
  • TBD

[olivergillespie]

Accidentally closed by deleting the branch on my fork. Opened #531 instead.