Use SSize_t/STRLEN in more places in regexp code

Use SSize_t/STRLEN in more places in regexp code

As part of getting the regexp engine to handle long strings, this com-
mit changes any variables, parameters and struct members that hold
lengths of the string being matched against (or parts thereof) to use
SSize_t or STRLEN instead of [IU]32.

To avoid having to change any logic, I kept the signedness the same.

I did not change anything that affects the length of the regular
expression itself, so regexps are still practically limited to
I32_MAX.  Changing that would involve changing the size of regnodes,
which would be a lot more involved.

These changes should fix bugs, but are very hard to test.  In most
cases, I don’t know the regexp engine well enough to come up with test
cases that test the paths in question with long strings.  In other
cases I don’t have a box with enough memory to test the fix.