bignum.c: suppress warnings

* bignum.c (big_shift2, rb_big_lshift, rb_big_rshift): cast explicitly
  to suppress warnings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41978 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* bignum.c (rb_big_coerce): [DOC] Add docs for Bignum#coerce
  Based on patch by Juanito Fatas [Fixes GH-360]
  https://github.com/ruby/ruby/pull/360


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41979 b2dd03c8-39d4-4d8f-98ff-823fe69b080e