BitArray API refinements & additions

[lorentey]

This is a small step towards making this ready for API review.

• BitArray now prints as if it contained a numeric value in binary representation. This seems the far more logical choice, given the BinaryInteger conversion methods and shift semantics below, and it also matches std::bitset.
• BitArray now conforms to ExpressibleByStringLiteral and LosslessStringConvertible, with the same format.
• We now have a family of generic BinaryInteger initializers that take BitArray values.
• New bitwise operations: toggleAll(), toggleAll(in:).
• New array operations: append(repeating:count:), insert(repeating:count:at:).
• New shift operations: resizingShiftLeft(by:), resizingShiftRight(by:), maskingShiftLeft(by:), maskingShiftRight(by:).
• Doc updates.

Obvious to do items still remaining:

• Masking shifts within a slice of a bit array (incl. range expressions)
• Bit rotations, supporting rotating the entire array as well as rotating bits in a specific slice
• Bit reversals, supporting reversing the entire array as well as reversing bits in a specific slice
• Review & revise bitwise operations. It ought to be possible to e.g. to do a bitwise OR over bit array slices.
• Tests, benchmarks, etc.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test

[lorentey]

Landing this to set the stage for a potential RFC on the project forum. Filed #274 to track missing functionality in BitArray.