[stdlib] Eliminate _HasherCore protocol

[lorentey]

(This is a spinoff of #20185; it was created because eliminating _HasherCore used to cause a code size regression.)

Hasher only supports a single algorithm, so _HasherCore is an unnecessary protocol. It wasn't ever intended to be ABI-visible; the expectation was that Hasher would become a resilient struct, and internal details wouldn't become hardcoded in the ABI.

Unfortunately Hasher resiliency did not happen, so we're setting its internal layout & types in stone. This PR sanitizes these internals, updating them to the brave new fragile Hasher world.

Types that are implementation details of Hasher are now tucked away inside it, unnecessary generics are eliminated, names are made more sensible:

Old name	New name
protocol _HasherCore	(removed)
struct _HasherTailBuffer	struct Hasher._TailBuffer
struct _BufferingHasher<Core>	struct Hasher._Core
struct Hasher._Core	merged into struct Hasher._State

• Hasher._State is low-level SipHash implementation; it includes the fragile hashing state and internal operations that update it.
• Hasher._TailBuffer is a small buffer collecting bytes into word-size chunks that can be fed to _State.
• Hasher._Core consists of a tail buffer and a state; it implements low-level Hasher operations. (It's a little like _StringGuts.) Public Hasher operations call into it.

[lorentey]

@swift-ci benchmark

[lorentey]

With the new String representation, this is likely a viable change now.

[lorentey]

@swift-ci please test

[lorentey]

@swift-ci please benchmark

[swift-ci]

Build comment file:

No performance and code size changes

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

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[lorentey]

@swift-ci test

[lorentey]

@swift-ci please smoke test and merge

[lorentey]

@swift-ci smoke test and merge