[SE-0163] Migrate from deprecated Unicode APIs (part 2)

[benrimmington]

Most of the changes are simple renames:

• UTF8 → Unicode.UTF8
• UTF16 → Unicode.UTF16
• UTF32 → Unicode.UTF32
• UnicodeScalar → Unicode.Scalar
• UnicodeCodec → Unicode.Encoding

Follow-up to: #32920

[benrimmington]

@swift-ci Please smoke test

[benrimmington]

@swift-ci Please smoke benchmark

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
String.data.Medium	85	96	+12.9%	0.89x (?)
ObjectiveCBridgeStubToNSDate2	490	550	+12.2%	0.89x (?)
Improvement	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	5600	4210	-24.8%	1.33x (?)

Code size: -O

Performance: -Osize

Improvement	OLD	NEW	DELTA	RATIO
DataReplaceMedium	4000	3700	-7.5%	1.08x (?)

Code size: -Osize

Performance: -Onone

Improvement	OLD	NEW	DELTA	RATIO
ObjectiveCBridgeStubToNSDate2	650	560	-13.8%	1.16x (?)

Code size: -swiftlibs

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: 8-Core Intel Xeon E5
  Processor Speed: 3 GHz
  Number of Processors: 1
  Total Number of Cores: 8
  L2 Cache (per Core): 256 KB
  L3 Cache: 25 MB
  Memory: 64 GB

[benrimmington]

@milseman This pull request uses the new names, but it doesn't deprecate the old ones.

[milseman]

Could you clarify what your goal is? This doesn't seem like an improvement to the readability of stdlib code.