raname package to diff_test

Doing this does not provide much of a benefit other than
exlicitly testing API usage. Also, this changes makes it more obvious
to call diff.Runes and diff.Ints in their respective tests
(as opposed to Diff itself on &ints{} and &runes{}.

I did have to pull the type ints struct from the diff.go file for the
BenchmarkDiff call itself, though.
add diff.Strings
added TestDiffStrings

This is basically a duplicate of TestDiffRunes:
because the rune slice was all ascii, rune indices
line up with string indices
add BenchmarkDiffBytes and BenchmarkDiffStrings

benchmarks show diff.Bytes and diff.Strings almost equal
in timings (as expected