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Issue Description:

Unpredictable behaviour in the case of inserting class instance multiple times in set.
When a set was initialised as shown below then we can get 2 situations:

let a = NewClass(x: 1)
var set = Set<NewClass>()
set.insert(a)
a.x = 2
set.insert(a)
print(set.map { $0.x })

1) It is all ok and it prints:

[2]

2) Fatal error

Fatal error: Duplicate elements of type 'NewClass' were found in a Set.
This usually means either that the type violates Hashable's requirements, or
that members of such a set were mutated after insertion.

When a set was initialised from an array as shown below then we can get numerous situations:

let a = NewClass(x: 1)
var array: Array<NewClass> = [a, a]
a.x = 2
var set = Set(array)
a.x = 1
set.insert(a)
a.x = 3
set.insert(a)
print(set.map { $0.x })

It prints either [3] or [3, 3] or even [3, 3, 3].

I also understand that the changing of class's property that is take place in hash formation is unacceptable.

Attach 2 files with source code and compiled files if needed.