[Bug #21380] Prohibit modification in String#split block

	Reported at https://hackerone.com/reports/3163876

	Check negative integer underflow

	Many of Oniguruma functions need valid encoding strings

Fix memory leak in String#start_with? when regexp times out

[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

	[Bug #20292] Truncate embedded string to new capacity

Fix coderange of invalid_encoding_string.<<(ord)

	Appending valid encoding character can change coderange from invalid to valid.
	Example: "\x95".force_encoding('sjis')<<0x5C will be a valid string "\x{955C}"

Fix memory leak in grapheme clusters

	[Bug #20150]

	String#grapheme_cluters and String#each_grapheme_cluster leaks memory
	because if the string is not UTF-8, then the created regex will not
	be freed.

	For example:

	    str = "hello world".encode(Encoding::UTF_32LE)

	    10.times do
	      1_000.times do
	        str.grapheme_clusters
	      end

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

	Before:

	    26000
	    42256
	    59008
	    75792
	    92528
	    109232
	    125936
	    142672
	    159392
	    176160

	After:

	    9264
	    9504
	    9808
	    10000
	    10128
	    10224
	    10352
	    10544
	    10704
	    10896
	---
	 string.c                 | 98 +++++++++++++++++++++++++++++++-----------------
	 test/ruby/test_string.rb | 11 ++++++
	 2 files changed, 75 insertions(+), 34 deletions(-)

The test fails when RGENGC_CHECK_MODE is turned on:

    TestString#test_sub_gc_compact_stress = 9.42 s
    1) Failure:
    TestString#test_sub_gc_compact_stress [test/ruby/test_string.rb:2089]:
    <"aaa [amp] yyy"> expected but was
    <"aaa [] yyy">.