The pool was storing strings and references to it. References
became invalid as strings got moved around, as a consequence of
push_back() in the owner container. I'm storing references in
a custom StrRange struct now, and string refs are built on the
fly when one is needed.
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.
It's still missing ast optimization and some code cleanup is also
needed, but it should be working in the general case.
Also write a small unit test for it.
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.
