add strings module

The most common splitting in cohttp falls in two 2 categories:

- unbounded split on a single char
- bounded split on a single char

this module provides both functionalities through the split function
add tests for internal strings module
update _oasis

- mark Strings as internal module
- Add a test section for strings' tests
oasis regen
uses Strings.split where possible
remove module aliasing of Re_str

It was used for only a single function. Makes it harder to grep for
things
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
Merge pull request #100 from rgrinberg/re_str_removals

Re str removals