Recently I needed to implement `contains` methods to check if a range
contains another range. I think those are basic operations and suitable for
the standard library.

Although it may seem easy to implement such `contains` methods whenever we
need them, their precise specifications are too complicated to do so.


let a: ClosedRange<Int> = 2...7
a.contains(3...5) // `true`
a.contains(3...7) // also `true`
a.contains(3..<8) // still `true` because all values contained in `3..<8`
are also in `a`
a.contains(3..<9) // `false`

let b: ClosedRange<Float> = 2...7
b.contains(3...5) // `true`
b.contains(3...7) // `true`
b.contains(3..<8) // `false` because { x | 7.0 < x < 8.0 } is not contained
in `a`

let c: Range<Float> = 2..<7
c.contains(3...5) // `true`
c.contains(3..<7) // `true`
c.contains(3...7) // `false` because 7.0 is not contained in `a`

My experimental implementation is here:
(Currently does not support one-sided ranges)

What are your thoughts about them?

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