[stdlib] Force-inline some Sequence/Collection customization points

[lorentey]

This should be a code size win for specialized code at least, but I suspect there may be some performance improvements, too.

[lorentey]

@swift-ci smoke benchmark

[lorentey]

@swift-ci smoke test

[swift-ci]

Build comment file:

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Improvement
DictionaryKeysContainsNative	45	40	-11.1%	1.12x

Code size: -O

TEST	OLD	NEW	DELTA	RATIO
Improvement
DictionaryKeysContains.o	18963	17043	-10.1%	1.11x

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
UTF8Decode_InitFromBytes_ascii	490	582	+18.8%	0.84x
Improvement
Array2D	13194	11998	-9.1%	1.10x

Code size: -Osize

TEST	OLD	NEW	DELTA	RATIO
Improvement
DictionaryKeysContains.o	16859	15003	-11.0%	1.12x
NibbleSort.o	20000	19616	-1.9%	1.02x

How to read the data

The tables contain differences in performance which are larger than 8% and differences in code size which are larger than 1%.

If you see any unexpected regressions, you should consider fixing the regressions before you merge the PR.

Noise: Sometimes the performance results (not code size!) contain false alarms. Unexpected regressions which are marked with '(?)' are probably noise. If you see regressions which you cannot explain you can try to run the benchmarks again. If regressions still show up, please consult with the performance team (@eeckstein).

Hardware Overview

  Model Name: Mac Pro
  Model Identifier: MacPro6,1
  Processor Name: 12-Core Intel Xeon E5
  Processor Speed: 2.7 GHz
  Number of Processors: 1
  Total Number of Cores: 12
  L2 Cache (per Core): 256 KB
  L3 Cache: 30 MB
  Memory: 64 GB

---

[lorentey]

Results are as expected. Would've loved to see movement beyond Dictionary.Keys, although I guess that is the primary benefactor right now.