Update: Add f_conversion unit tests.

Update: Add f_conversion unit tests.

There are far more permutations than what are handled within this commit.
Focus on only the most basic set of checks to write the unit tests for.

There are problems with mocking fwrite_unlocked() via the wrap strategy used by the linker.
I do not know why this is not working so I commented out the code and moved on.

Several problems are exposed and are solved.
- This exposed the fwrite_unlocked() return results problems.
- Add support for big endian (untested).
- Zero values are not correctly built (see below).

Make sure to count the zero number as a single digit.
Get rid of the "used" count and rely only on the digits.
Do not include the 0 digit when determining the padding.
Change the prefix append function to accommodate 0 and consistently call the prefix append function.

Update documentation comments.