add nqp::eqat to jvm and parrot.
special case same-encoding strings, length-1 needles.
STRING_ord takes chars, not bytes. local var thinko.
fix comment.
add docs for eqat opcode
fix eqat bounds check for good.
special-case len-1 needles in the parrot qast compiler
map eqat on moarvm
Merge branch 'eqat_unused'

This branch gives us an eqat op that checks if a substring of the
haystack is equal to the needle at a given offset. Works on all three
backends.

There are changes lying in wait for when there's a new stage0 that has
these ops in it, but it's not that important for now.