avoid variable shadowing

[german-one]

Even with cryptic variable names we cannot fully avoid to shadow variables in user code. However, we absolutely can use different variable names in the API to avoid shadowing when macros are nested.

[german-one]

Reviewing this PR might be easier with "Hide whitespaces" checked.
https://github.com/eteran/c-vector/pull/83/files?diff=unified&w=1

[eteran]

I'll have to take a look. Looks like a lot of small changes so may take a little bit 👍.

[german-one]

I forgot to mention that this doesn't just address potential issues.
That's what I get when I compile the unit tests:

cv_sz__ declared in cvector_resize is shadowed in cvector_grow via cvector_reserve.

Admittedly, instead of thinking long and hard about under which circumstances further warnings of that kind could be triggered, I took the pragmatic approach and simply gave all variables a unique name.

[...] Looks like a lot of small changes [...]

Yes, really only renaming of variables declared in the macros. With whitespaces hidden it's easier to spot.

[eteran]

Looks great, thanks!