Running gsub! 5 times with string arguments seems to be faster than
running it once with a regex and Hash.

When there are matches to the regex (there are characters to escape)
this is faster in part because CRuby will allocate a new match object
and string as a key to lookup in the map hash provided. It's possible
that could be optimized upstream, but at the moment this avoids those
allocations.

Surprisingly (at least to me) this is still much faster when there is no
replacement needed: in my test ~3x faster on a short ~200 byte string,
and ~5x faster on a pre-escaped ~600k twitter.json.