Improve AttributedString.CharacterView to Substring conversion in AttributedStringProtocol.range(of:)

[jmschonfeld]

We don't currently have range(of:) implemented natively on BigString (or a UTF-8 collection) and instead we must first convert to a Substring to perform the searching operation. In the future, we should benchmark and improve this function by writing it natively on the BigString storage instead of bridging to String. However, today we're taking a very slow, linear path of converting the character view to a Substring iterating character by character. Instead, we can take a faster path by using our CharacterView to String conversion that copies chunk-by-chunk and create a Substring from that result. The benchmarks show that this has a significant performance improvement over the existing implementation (with much more room to improve with a native implementation)

AttributedStringBenchmarks

range(of:) metrics

Time (wall clock): results within specified thresholds, fold down for details.

Time (wall clock) (ns) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	3745	3963	4198	4321	4366	4444	4613	240
branch	375	403	439	449	458	486	554	2296
Δ	-3370	-3560	-3759	-3872	-3908	-3958	-4059	2056
Improvement %	90	90	90	90	90	89	88	2056

Time (total CPU): results within specified thresholds, fold down for details.

Time (total CPU) (ns) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	3747	3965	4182	4325	4362	4432	4615	240
branch	376	404	440	450	458	481	534	2296
Δ	-3371	-3561	-3742	-3875	-3904	-3951	-4081	2056
Improvement %	90	90	89	90	90	89	88	2056

Throughput (# / s): results within specified thresholds, fold down for details.

Throughput (# / s) (K)	p0	p25	p50	p75	p90	p99	p100	Samples
main	267	252	238	231	229	225	217	240
branch	2665	2481	2277	2229	2185	2059	1806	2296
Δ	2398	2229	2039	1998	1956	1834	1589	2056
Improvement %	898	885	857	865	854	815	732	2056

Resolves rdar://155952423

[jmschonfeld]

@swift-ci please test