* [stdlib][SR-13883] Avoid advancing past representable bounds when striding.
* [stdlib] Expand a test and add a comment to ensure correct floating-point stride bounds checking.
* [stdlib][NFC] Clarify a comment in a test.
* [stdlib][NFC] Adjust copyright notices, clarify comments, delete '-swift-version=3' for tests.
* [stdlib] Add implementations for fixed-width integer strides for performance.
* [stdlib] Document `Strideable._step` and modify overflow checking behavior of `Stride*Iterator`.
* [stdlib] Address reviewer comments, postpone documentation changes
* [stdlib][NFC] Update documentation for '_step(after:from:by:)'
* [stdlib][NFC] Use 'nil' instead of an arbitrary value for integer striding '_step' index
