internal.h: move inline functions

* internal.h (rb_float_value, rb_float_new): move inline functions
  from ruby/ruby.h.
* numeric.c (rb_float_value, rb_float_new): define external functions
  for extension libraries.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43042 b2dd03c8-39d4-4d8f-98ff-823fe69b080e