Remove typedefs for float_t and double_t

The actual float and double data types are defined in C89 and are
usable as such without any extra typedefs.

Removing the extra typedefs simplifies the compatibility typedef
headers, simplifies portability and makes the code base easier
to work with for people new to the library.
Remove the real32_t typedef

The float type is defined in standards and is portably available
on all supported compilers and platforms.
Merge pull request #269 from mstorsjo/remove-float-typedefs

Remove typedefs for float_t, double_t and real32_t