Un-unified `??=` & `=??`
Un-unified `??=` & `=??`

Decided that there is a strong distinction between `??=` & `=??`— when both the LHS & RHS are non-nil, the former will keep the LHS value, while the latter will take the RHS value.  Both variants are valuable in different use-cases, and I can think of a way to combine `??=` with some other Swift operator(s) to get succinct syntax for both use-cases.

* Added back `=??` operator variants _(taking `lhs`s of type `Wrapped`, `Wrapped?`, or `Wrapped!`)_ that only check if `rhs != nil` in determination of whether to assign.  
	‣ Also supporting only variants that make sense— `=??`'s RHS must be optional, and `??=`'s LHS must be optional.
* Commented-out `??=` variants with `rhs…`s that return non-optional `Wrapped` values, because Swift complains _Ambiguous use of operator '??='_ when using a closure that returns a never-optional `Wrapped`.  
	‣ It's almost as if Swift is auto-promoting `Wrapped`→`Wrapped?` and then treats them with equal priority in determining which override to use.  Seems like a Swift compiler bug to me.  
	‣ The only alternate solution is to explicitly type the closures _(via `{ () -> Wrapped … }()` or `{ … }() as Wrapped`)_, which is a huge PitA and goes completely against the point of these operators— brevity.
* Improved `=??` & `??=` function doc-comments.
capnslipp/NilCoalescingAssignmentOperators,capnslipp/NilCoalescingAssignmentOperators