[swift-evolution] [SR-933] Rename flatten to flattened

[Brent Royal-Gordon]

>> Do you also think that trigonometry should be `foo.sined`, `foo.cosined`, and `foo.tangented`?
> 
> sine, cosine etc. are all nouns so I think as computed properties they’d be fine as is?

So you would favor these?

	_ = number.sine()
	_ = number.cosine()
	_ = number.tangent()
	
	number.formSine()
	number.formCosine()
	number.formTangent()

-- 
Brent Royal-Gordon
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