* 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
This PR updates `_FixedArray${N}` to conform to `ExpressibleByIntegerLiteral` rather than `IntegerLiteralConvertible`. This silences a compiler warning (actually, several).
Adds a internal-only fixed size array, which we will be using in
String as a buffer of code units. This is basically just a wrapper
around a tuple that provides a random access collection
interface. This is gyb-ed up to scale up to arbitrary sizes, as
specified by the "sizes" gyb variable.
