* Add constraints to FixedWidthInteger.Stride and .Magnitude
Add the constraint that these associatedtypes themselves conform to FixedWidthInteger and to SignedInteger and UnsignedInteger, respectively. These are effectively part of the semantic requirement of the protocol already, because the requirements on .min and .max effectively force FixedWidthInteger to be twos-complement, which requires Magnitude not be Self for signed types. Also, in practice it's generally necessary to have these two constraints in order to effectively use the FixedWidthInteger protocol anyway; witness that by adding them to the protocol, we eliminate them as constraints from a number of extensions.
This also resolves https://bugs.swift.org/browse/SR-8156
Deprecated the `PlaygroundQuickLook` enum and `CustomPlaygroundQuickLookable`
protocol. These are being targeted for removal in Swift 5, so we want to
unconditionally deprecate them now to encourage use of
`CustomPlaygroundDisplayConvertible` instead.
This commit includes deprecated the various `CustomPlaygroundQuickLookable`
conformances across the standard library and overlay libraries.
...as detected by initializing an individual field without having
initialized the whole object (via `self = value`).
This only applies in pre-Swift-5 mode because the next commit will
treat all cross-module struct initializers as delegating in Swift 5.
I failed to merge the upstream changes to swift-corelibs-foundation at the same
time as I merged that #9806, and it broke on linux. Going to get it right this
time.
