add the trivial strings_after function
add various little helpers to create specialized parsers

for headers, cookies
add cookie/header splitters
remove useless creation of re

this one was not being used
make cookie/header use specialized parsers
get_media_type now uses Re directly instead of Re_str

not the fastest but at least it's threadsafe (but very ugly)
add printer to media type test
lift Str.quote from ocaml-re

This commit might not be completely necessary but with it we still
need Re_str present. This way we can be 100% Re_str free.
rename Strings -> Stringext