Another approach to formatting floating points as strings

The printf syntax with format specs such as '%+5.2f' used by C/python/... is
hard to read.  Instead, this code provides a key--value syntax, expandably.
Example:

    "\fp_format:nn { 123.4567 } { precision = 5 , width = 9 , fill = 0 }"

expands to "000123.46".


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5275 de43f980-851b-0410-b2f7-c40aca1f87e0