Issue Description:

To enable custom implementations of FixedWidthInteger, and to speed up and simplify generic code that needs to initialize integer types (especially those that are wider than UInt64) from arbitrary data, we should add dedicated initializers to BinaryInteger that take a specific word or a sequence of words in two's complement representation:

protocol BinaryInteger {
  init(truncatingWordIfNeeded word: UInt)
  init<W: Sequence>(truncatingWordsIfNeeded words: W) where W.Element == UInt
}

The new initializers should be public.

The single-word initializer is only there to provide a potential speed boost for the most common case. For FixedWidthInteger (and possibly also BinaryInteger itself), it seems possible to provide a default implementation for the sequence-based initializer using the single-word initializer and shifts, but it seems better to leave init(truncatingWordsIfNeeded: ) as a required initializer. (The default implementation would be quadratic for most big integer types.)

Note that for FixedWidthInteger, we currently require the underscored initializer FixedWidthInteger.init(_truncatingBits bits: UInt), without providing a default implementation. This makes it impossible to implement FixedWidthInteger using just the public API.