Rework binary integer description error-handling

[oscbyspro]

This patch reworks binary integer description error-handling using the Optional<Fallible<T>> approach (#95). In other words: invalid inputs return nil, and out-of-bounds inputs return the truncated results and an error indicator—like basically every other BinaryInteger operation. Note that only TextInt exposes the new functionality and that the methods on BinaryInteger replace error with nil. Additionally, this patch removes TextInt.Error (#125) and all StaticString conveniences (awaiting a unified abstraction). It is also a bit faster.

[oscbyspro]

-&0 is (value: 1, error: true) for all binary integer types, so there are no unchecked() convenience methods.