SE-0138: Proposed amendment to SE-0138: Normalize UnsafeRawBufferPointer Slices (#8222)

* Add sliceability tests for Unsafe(Raw)BufferPointer.

Improve the generic sliceability tests to verify that SubSequence indices are
compatible with their parents indices.

* Fix and enable testing stdlib Collection instances.

Top-level entry points fully testing a collection instance:
check${Traversal}Collection

One level of recursion into all slices of the collection instance
O(n^2). (Not combinatorial).

Previously, checkCollection() did nothing. So much of the testing infrastructure was inactive. Now it runs all forward collection tests.

Fixes a bug in subscriptRangeTests.

The UnsafeRawBufferPointer and Data collection testing is disabled and
will be fixed in the following commit.

* Give UnsafeRawBufferPointer a distinct slice type.

SubSequence = RandomAccessSlice<Self>

* Fix raw buffer pointer tests after changing the API

* Add UnsafeRawBuffer(rebasing:) initializers.

Allows converting a raw slice into a zero-based raw buffer,
which is a common operation on flat memory.

Add and update UnsafeRawBufferPointer unit tests.

* Do not run recursive O(n^2) collection slice testing on large collections.

Now, even with collection unit testing wired up, the validation tests
take the same amount of time to execute.

* Add init(rebasing:) to UnsafeBufferPointer.

This is required for consistency with UnsafeRawBufferPointer.

* Update CHANGELOG.md for SE-0138 amendment: UnsafeRawBufferPointer slice type.

validation-test/stdlib/UnicodeTrie.swift.gyb

@@ -144,7 +144,7 @@ func checkGraphemeClusterSegmentation(
   )
 
   let expectedCharacters: [Character] = Array(subject.characters)
-  checkSliceableWithBidirectionalIndex(expectedCharacters, subject.characters)
+  checkBidirectionalCollection(expectedCharacters, subject.characters)
 }
 
 func checkGraphemeClusterSegmentation(