perf impact of format_err!(..) in decode_buffer tight loop

[jjhbw]

Hi! Thanks for these useful bindings.

I noticed that the use of format_err! slows down the tight loop in decode_buffer quite a bit. Considering it is just a static error string it performs a lot of redundant work in a tight loop and it should be trivial to refactor out. I would make a PR myself, but I couldn't get the error types to line up with the rest of the library in time.

The line in question:

openslide-rust/src/utils.rs

Line 99 in 2993aac

.ok_or(format_err!("Conversion to primitive error"))?

A flamechart of the current situation:

After turning the format_err! call to a static error string (copied the whole thing into my project's codebase):