This also fixes a subtle bug: the size passed to MaxSizedArray in
int_to_string() was calculated on the max value of type F, but when
F was a signed type the max would only be for example 31 bits.
Conversion of negative numbers would then fail with an assertion.
They are useful in the Redis backend and in helpers itself.
People wanting to use pq alone from this project will have to cope and
search for all the needed files.
This was needed so that machinery wouldn't depend on common.
Also moved compatibility.h to helpers include dir.
Ideally mediatypes.hpp will be merged into mediatype.hpp, or there
will be some kind of renaming because the two names are
getting really confusing.
