[Bug #20653]

This commit refactors how Onigmo handles timeout. Instead of raising a
timeout error, onig_search will return a ONIGERR_TIMEOUT which the
caller can free memory, and then raise a timeout error.

This fixes a memory leak in String#start_with when the regexp times out.
For example:

    regex = Regexp.new("^#{"(a*)" * 10_000}x$", timeout: 0.000001)
    str = "a" * 1000000 + "x"

    10.times do
      100.times do
        str.start_with?(regex)
      rescue
      end

      puts `ps -o rss= -p #{$$}`
    end

Before:

    33216
    51936
    71152
    81728
    97152
    103248
    120384
    133392
    133520
    133616

After:

    14912
    15376
    15824
    15824
    16128
    16128
    16144
    16144
    16160
    16160

These methods return the byte-based offset of the beginning or end of the specified match.

[Feature #20576]

[Feature #18576]

Since outright renaming `ASCII-8BIT` is deemed to backward incompatible,
the next best thing would be to only change its `#inspect`, particularly
in exception messages.

[Bug #20228]

rb_reg_onig_match can raise a Regexp::TimeoutError, which would cause
the OnigRegion to leak.

[Bug #20228]

If rb_reg_check_timeout raises a Regexp::TimeoutError, then the stk_base
will leak.

Please consider using misc/expand_tabs.rb as a pre-commit hook.

Fixes [Bug #20104]