Similar to the previous commit, this simplifies future refactoring
to change the way edges are stored, and further differentiates
single poly variables from array pointers.
With the goal of clearly differentiating between arrays and single
elements, improving consistency across Blender, and using wording
that's easier to read and say, change variable names for Mesh edges
and polygons/faces.
Common renames are the following, with some extra prefixes, etc.
- `mpoly` -> `polys`
- `mpoly`/`mp`/`p` -> `poly`
- `medge` -> `edges`
- `med`/`ed`/`e` -> `edge`
`MLoop` variables aren't affected because they will be replaced
when they're split up into to arrays in #104424.
Using spans instead of raw pointers helps to differentiate ararys from
pointers to single elements, gives bounds checking in debug builds, and
conveniently stores the number of elements in the same variable.
Also make variable naming consistent. For example, use `loops` instead
of `mloop`. The plural helps to clarify that the variable is an array.
I didn't change positions because there is a type mismatch between
C and C++ code that is ugly to manage. All remaining code can be
converted to C++, then that change will be simpler.
Pull Request #105138
