DRAFT: Fast paths for large, native strings

[milseman]

Pulls in a bunch of String-internal bit fiddly code in order to quickly get a pointer to contiguous UTF-8 if we're a (large) native string.

Extract out the Metrics data (unused in release builds), as it seems like having Processor surpass the 256-byte size causes significant regressions for some reason. (see milseman#1).

The below results are post- #642 and post- #644

=== Regressions ======================================================================
- EmojiRegexAll                           72.9ms	71.9ms	1.03ms		1.4%
- SubtractionCCC                          21.7ms	21.3ms	314µs		1.5%
- BasicCCC                                10.8ms	10.5ms	261µs		2.5%
- CaseInsensitiveCCC                      12ms	11.7ms	255µs		2.2%
- IntersectionCCC                         22.1ms	21.9ms	206µs		0.9%
- BasicRangeCCC                           11.1ms	11ms	196µs		1.8%
- IPv4Address                             2.64ms	2.54ms	99.2µs		3.9%
=== Improvements =====================================================================
- CompilerMessagesAll                     113ms	116ms	-2.71ms		-2.3%
- DiceRollsInTextAll                      47.3ms	49.6ms	-2.23ms		-4.5%
- AnchoredNotFoundWhole                   7.44ms	9.18ms	-1.74ms		-18.9%
- NotFoundAll                             6.29ms	7.09ms	-797µs		-11.2%
- EmailLookaheadAll                       39.3ms	39.9ms	-683µs		-1.7%
- EmailBuiltinCharacterClassAll           14.8ms	15.5ms	-663µs		-4.3%
- ReluctantQuantWithTerminalWhole         8.77ms	9.3ms	-525µs		-5.6%
- LiteralSearchAll                        6.17ms	6.65ms	-478µs		-7.2%
- HangulSyllableAll                       6.35ms	6.76ms	-412µs		-6.1%
- LiteralSearchNotFoundAll                6.05ms	6.42ms	-369µs		-5.8%
- GraphemeBreakNoCapAll                   5.26ms	5.52ms	-264µs		-4.8%
- WordsAll                                14.2ms	14.5ms	-264µs		-1.8%
- symDiffCCC                              48.7ms	49ms	-240µs		-0.5%
- EmailLookaheadList                      9.65ms	9.88ms	-230µs		-2.3%
- CssAll                                  3.63ms	3.85ms	-222µs		-5.8%
- EmailLookaheadNoMatchesAll              40.6ms	40.8ms	-204µs		-0.5%
- HangulSyllableFirst                     3.06ms	3.25ms	-194µs		-6.0%
- DiceNotation                            5.21ms	5.39ms	-179µs		-3.3%
- BasicBuiltinCharacterClassAll           9.02ms	9.13ms	-108µs		-1.2%
- EagarQuantWithTerminalWhole             2.55ms	2.61ms	-62.8µs		-2.4%
- LinesAll                                3.05ms	3.1ms	-42.7µs		-1.4%
- MACAddress                              3ms	3.02ms	-24.7µs		-0.8%

[milseman]

TODO: investigate re-enabling this code, or even consider looking at all potential callers of loadScalar.

[milseman]

TODO: add assertion to check fast-path produces same result as slow path

[milseman]

TODO: remove

[stephentyrone]

Not sure why that got marked as a review. Ignore.