internal.h: STATIC_ASSERT

* internal.h (STATIC_ASSERT): move from enum.c.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41895 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
bignum.c: sizeof_bdigit_dbl

* bignum.c (sizeof_bdigit_dbl): check sizeof(BDIGIT_DBL).

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41896 b2dd03c8-39d4-4d8f-98ff-823fe69b080e