Tidied up sliding windows, minor cosmetic nitpicks

Tidied up sliding windows, minor cosmetic nitpicks

Added `static` to the sliding window functions, reworked those functions
a bit to improve the (internal) API.  Simplified the double scalarmult
accordingly.

Added FOR_T macro, for when the index should be a type other than
size_t.  Helped remove explicit conversions in Argon2i and sliding
windows.  Hopefully this new macro will be obvious to reviewers.  I
could have used the regular `for` loop, but it took too much horizontal
space in Argon2i (we use long names there).