Add \dim_to_pt:n and \dim_to_unit:nn as candidate functions.

Right now, those functions use TeX's \dimexpr to produce
a dimension, then ignore anything left after the trailing
'pt', just like \__dim_strip_pt:n does.

The \dim_to_unit:nn and \dim_ratio:nn functions are very
similar: one returns a decimal number, the other one a
rational fraction.  Perhaps \dim_ratio:nn should also
ignore trailing things, or \dim_to_unit:nn should not.

Comments welcome.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4486 de43f980-851b-0410-b2f7-c40aca1f87e0