Add proposal for generalizing ContiguousBytes to support Span

[DougGregor]

API proposal corresponding to #1565

[Jnosh]

Suggested change

in the protocol. However, not all types that currently conform to the `ContiguousBytes` protocol can provide a `bytes` property that satisfies this. For example, a type that needs to materialize data into a buffer to pass to the closure provided to `with(Unsafe)Bytes` would not be able to implement this property, which depends on having the lifetime of the resulting `RawSpan` tied to that of its enclosing type. [SE-0456](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0456-stdlib-span-properties.md) describes some of the changes required in the implementations of `String.UTF8View` and `Array` that were needed to provide `span` properties and which might not be possible for other types. Therefore, while adding this property would provide more ergonomic access to the contiguous bytes of a type, doing so necessary breaks source compatibility.

in the protocol. However, not all types that currently conform to the `ContiguousBytes` protocol can provide a `bytes` property that satisfies this. For example, a type that needs to materialize data into a buffer to pass to the closure provided to `with(Unsafe)Bytes` would not be able to implement this property, which depends on having the lifetime of the resulting `RawSpan` tied to that of its enclosing type. [SE-0456](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0456-stdlib-span-properties.md) describes some of the changes required in the implementations of `String.UTF8View` and `Array` that were needed to provide `span` properties and which might not be possible for other types. Therefore, while adding this property would provide more ergonomic access to the contiguous bytes of a type, doing so necessarily breaks source compatibility.

[DougGregor]

@swift-ci please test