* benchmark/bm_io_nonblock_noex2.rb: new benchmark based
  on bm_io_nonblock_noex.rb
* io.c (io_read_nonblock): move documentation to prelude.rb
  (io_write_nonblock): ditto
  (Init_io): private, internal methods for prelude.rb use only
* prelude.rb (IO#read_nonblock): wrapper + documentation
  (IO#write_nonblock): ditto
  [ruby-core:71439] [Feature #11339]

rb_scan_args and hash lookups for kwargs in the C API are clumsy and
slow.  Instead of improving the C API for performance, use Ruby
instead :)

Implement IO#read_nonblock and IO#write_nonblock in prelude.rb
to avoid argument parsing via rb_scan_args and hash lookups.

This speeds up IO#write_nonblock and IO#read_nonblock benchmarks
in both cases, including the original non-idiomatic case where
the `exception: false' hash is pre-allocated to avoid GC pressure.

Now, writing the kwargs in natural, idiomatic Ruby is fastest.
I've added the noex2 benchmark to show this.

2015-11-12 01:41:12 +0000
target 0: a (ruby 2.3.0dev (2015-11-11 trunk 52540) [x86_64-linux])
target 1: b (ruby 2.3.0dev (2015-11-11 avoid-kwarg-capi 52540)
-----------------------------------------------------------
benchmark results:
minimum results in each 10 measurements.
Execution time (sec)
name              a       b
io_nonblock_noex    2.508   2.382
io_nonblock_noex2   2.950   1.882

Speedup ratio: compare with the result of `a' (greater is better)
name              b
io_nonblock_noex    1.053
io_nonblock_noex2   1.567

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52541 b2dd03c8-39d4-4d8f-98ff-823fe69b080e