doco rounding functions

[Kevin Ryde]

A bit of a table in the same style as the primitive numerics section,

        * scheme-data.texi (Arithmetic): Use a table for scheme to C libm
        equivalences, add C99 trunc.


   C functions for some of the above rounding functions are provided by
the standard C mathematics library.  Naturally these expect and return
`double' arguments (*note Rounding Functions: (libc)Rounding
Functions.).

     Scheme Procedure   C Function
     `floor'            `floor'
     `ceiling'          `ceil'
     `truncate'         `trunc'

   `trunc' is C99 standard and might not be available on older systems.
Guile provides an `scm_truncate' equivalent (on all systems), plus a C
level version of the `round' Scheme procedure.

 - C Function: double scm_truncate (double x)
 - C Function: double scm_round (double x)