`BitArray`/`BitSet` need a way to export the underlying bitmaps in some sensible format

We need a good way to extract the underlying bitmaps out of a `BitArray`/`BitSet` value in a reasonably efficient (and reasonably elegant) way. We also need a way to then convert that data back into a `BitArray`/`BitSet` value. `BitArray` and `BitSet` internally store their contents in what is effectively an array of UInt64, making it subject to endianness issues. This makes it unlikely we'd want them to expose their storage directly -- at least, not without overhauling these types to use a simpler representation. (Which is not out of the question, either.) If we want to preserve the current representation, one possible move is to offer conversions to/from raw buffer pointers (or, preferably, `RawSpan`/`OutputRawSpan`). ```swift extension BitArray { // And BitSet init(bitPattern: UnsafeRawBufferPointer) func exportBitPattern(initializing target: UnsafeMutableRawBufferPointer) init(bitPattern: RawSpan) func exportBitPattern(to target: inout OutputRawSpan) } ``` (Names are tricky, as usual.) These new APIs need to make a decision if they use a little- or big-endian representation for the byte buffers they operate on; alternatively, this can be a user-configurable input argument. The most elegant way of doing this would be to change the underlying representation of these types, and simply add a `var bitPattern: RawSpan` property that directly exposes their storage with O(1) complexity. (That too would need to make a decision on endianness; and in this case that cannot be user-configurable.)