string.c: reduce memory copy

* string.c (rb_str_lstrip, rb_str_strip): reduce memory copy by
  copying necessary part only.
* string.c (rb_str_strip_bang, rb_str_strip): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48277 b2dd03c8-39d4-4d8f-98ff-823fe69b080e