Change isize to i32

[moritz-gross]

I was looking to learn Rust a bit more and saw that isize is used a lot in chemistry.rs, though I don't see why it shouldn't be a standard integer type like i32.
as I understand it, isize is primarily for pointers, ie platform dependent, which does not make sense for stuff like the likely_chem_superscript scoring function.

side note: I find the RegEx's like MULTIPLE_PLUS_OR_MINUS_OR_DOT very hard to read. What do you think about having some intermediate parts to have it in smaller chunks?

[NSoiffer]

That looks good. I suspect at some point, I started with a usize, realized I needed negative numbers, so just went with isize. i32 makes more sense. Efficiency-wise (memory/speed), it is irrelevant.

There are probably some usize usages that might make sense to be u32, but again, I'm pretty sure I used u32 in all the spots that matter.