rename(2) requires two pathname resolution operations which can
take considerable time on slow filesystems, release the GVL so
operations on other threads may proceed.

On fast, local filesystems, this change results in some slowdown
as shown by the new benchmark.  I consider the performance trade
off acceptable as cases are avoided.

benchmark results:
minimum results in each 3 measurements.
Execution time (sec)
name	trunk	built
file_rename	2.648	2.804

Speedup ratio: compare with the result of `trunk' (greater is better)
name	built
file_rename	0.944

* file.c (no_gvl_rename): new function
  (rb_file_s_rename): release GVL for renames
* benchmark/bm_file_rename.rb: new benchmark

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60088 b2dd03c8-39d4-4d8f-98ff-823fe69b080e