io.c: recycle garbage on write

* string.c (STR_IS_SHARED_M): new flag to mark shared mulitple times
  (STR_SET_SHARED): set STR_IS_SHARED_M
  (rb_str_tmp_frozen_acquire, rb_str_tmp_frozen_release): new functions
  (str_new_frozen): set/unset STR_IS_SHARED_M as appropriate
* internal.h: declare new functions
* io.c (fwrite_arg, fwrite_do, fwrite_end): new
  (io_fwrite): use new functions

Introduce rb_str_tmp_frozen_acquire and rb_str_tmp_frozen_release
to manage a hidden, frozen string.  Reuse one bit of the embed
length for shared strings as STR_IS_SHARED_M to indicate a string
has been shared multiple times.  In the common case, the string
is only shared once so the object slot can be reclaimed immediately.

minimum results in each 3 measurements. (time and size)

Execution time (sec)
name                            trunk   built
io_copy_stream_write            0.682   0.254
io_copy_stream_write_socket     1.225   0.751

Speedup ratio: compare with the result of `trunk' (greater is better)
name    built
io_copy_stream_write            2.680
io_copy_stream_write_socket     1.630

Memory usage (last size) (B)
name                            trunk           built
io_copy_stream_write            95436800.000    6512640.000
io_copy_stream_write_socket     117628928.000   7127040.000

Memory consuming ratio (size) with the result of `trunk' (greater is better)
name    built
io_copy_stream_write            14.654
io_copy_stream_write_socket     16.505

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57469 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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