When compiling with -nostdlib, the functions in ctype may not be
available. Add FIXMATH_NO_CTYPE option to replace them with inline
functions.
Also add unit test to test str functions with this option enabled.
fix16_mul, fix16_div, fix16_sqrt.
They are faster & more accurate than the previous versions.
Closes issue #13.
Includes unittests for the functions in question, runnable
by typing 'make' in the unittests folder.