Speed up repeatcpy() by at least 40% for 1-char or numerous repeats

Speed up repeatcpy() by at least 40% for 1-char or numerous repeats

And don't make it receive the interpreter anymore.

For 1-char repeats, use memset(). Otherwise, use the old implementation up
to some (small) length, and then use memcpy() in a binary manner, based on
what we previously copied.

Note that we use memcpy() so both strings shouldn't overlap. The previous
implementation didn't allow this as well. This would be a good place to use
the restrict keyword from C99. I'm not sure if Configure has a probe for it.