Correctness: Make some private & internal classes sealed where possible

[Henr1k80]

Should in theory also be faster, as some virtual method table lookups can be avoided.
I haven't tested if it introduces any performance regressions due to runtime black magic inner workings.
My laptop cannot produce repeatable results due to throttling & background services.
I hope you have a benchmark setup to test it.

There are some whitespace changes due to:
trim_trailing_whitespace = true
insert_final_newline = true
in .editorconfig