New `=?` OptionalAssignOperator
New `=?` OptionalAssignOperator

* Created new `OptionalAssignOperator.swift` containing a `=?<Wrapped>(lhs:inout Wrapped, rhs:Optional<Wrapped>)` function, for making an assignment only if the `rhs` is non-nil.  
	‣ Basically, it's a `…=`-ification of the `??` operator— _`a =? b` means `a = b ?? a`_.

capnslipp/NilCoalescingAssignmentOperators,capnslipp/NilCoalescingAssignmentOperators