RUBY_DEBUG_LOG: Logging debug information mechanism (#3279)

* RUBY_DEBUG_LOG: Logging debug information mechanism

This feature provides a mechanism to store logging information
to a file, stderr or memory space with simple macros.

The following information will be stored.
  * (1) __FILE__, __LINE__ in C
  * (2) __FILE__, __LINE__ in Ruby
  * (3) __func__ in C (message title)
  * (4) given string with sprintf format
  * (5) Thread number (if multiple threads are running)

This feature is enabled only USE_RUBY_DEBUG_LOG is enabled.
Release version should not enable it.

Running with the `RUBY_DEBUG_LOG` environment variable enables
this feature.

  # logging into a file
  RUBY_DEBUG_LOG=/path/to/file STDERR

  # logging into STDERR
  RUBY_DEBUG_LOG=stderr

  # logging into memory space (check with a debugger)
  # It will help if the timing is important.
  RUBY_DEBUG_LOG=mem

RUBY_DEBUG_LOG_FILTER environment variable can specify the fileter string.
If "(3) __func__ in C (message title)" contains the specified string, the
infomation will be stored (example: RUBY_DEBUG_LOG_FILTER=str will enable
only on str related information).

In a MRI source code, you can use the following macros:
  * RUBY_DEBUG_LOG(fmt, ...): Above (1) to (4) will be logged.
  * RUBY_DEBUG_LOG2(file, line, fmt, ...):
    Same as RUBY_DEBUG_LOG(), but (1) will be replaced with given file, line.

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