Trigonometry in degrees, inverse trig, and clean up parsing

Add six trigonometric functions taking their arguments in
degrees, six arc-trigonometric functions yielding results
in radians, and six in degrees:
- cosd, cotd, cscd, secd, sind, tand
- acos, acot, acsc, asec, asin, atan
- acosd, acotd, acscd, asecd, asind, atand
This relies on some changes in l3fp-parse, which I cleaned
up a bit (mostly I removed some obsolete explanations in the
documentation).  Also some new internal functions to do
extended-precision computations.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4598 de43f980-851b-0410-b2f7-c40aca1f87e0