* Eradicate IndexDistance associated type, replacing with Int everywhere
* Consistently use Int for ExistentialCollection’s IndexDistance type.
* Fix test for IndexDistance removal
* Remove a handful of no-longer-needed explicit types
* Add compatibility shims for non-Int index distances
* Test compatibility shim
* Move IndexDistance typealias into the Collection protocol
Array, Set, Dictionary, and Optional all provide unconditional conformances
to Encodable & Decodable that dynamically check whether their type arguments
are Encodable/Decodable. Now that we have conditional conformances, make
these unconditional conformances properly conditional, removing all of
the Swift 4-era type-erasure hacks.
Fixes rdar://problem/34989162.
* Refactor Indices and Slice to use conditional conformance
* Replace ReversedRandomAccessCollection with a conditional extension
* Refactor some types into struct+extensions
* Revise Slice documentation
* Fix test cases for adoption of conditional conformances.
* [RangeReplaceableCollection] Eliminate unnecessary slicing subscript operator.
* Add -enable-experimental-conditional-conformances to test.
* Gruesome workaround for crasher in MutableSlice tests
* [SR-4005] Allow heterogenous comparisons in elementsEqual
When a user is supplying a predicate to compare the type equivalence
isn’t required
* elementsEqualWithPredicate tests
Compares a string of a number with an integer value by using the
elementsEqualPredicate closure
* Update test expectations to use new sequence element types
* Update hardcoded test to reference sequence
Conditional conformances aren't quite ready yet for Swift 4.1, so
introduce the flag `-enable-experimental-conditional-conformances` to
enable conditional conformaces, and an error when one declares a
conditional conformance without specifying the flag.
Add this flag when building the standard library (which will vend
conditional conformances) and to all of the tests that need it.
Fixes rdar://problem/35728337.
