SpecialFunctions moves to Numerics, beside the interval evaluation

The routines are the compilers' today, and the numerical evaluation's fast tier is where they
are to be shared next, as asked on #1607: the interval evaluation reads no special function, so an
integrator check with erf or Ei in it falls back to the decimal evaluation. Nothing but the
namespace and the file's place changes.

Part of #1607.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: Rafael-SOWNet