slices: relax the constraints to type whose underlying type is slice

[SilverRainZ]

Like what we done in exp/maps, exp/slices would be better to accept a
type whose underlying type is slice.

Due to golang/go#48619, constraints of quickSortOrdered and its
related functions are not changed. So cast S to []E for now.

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Message from Robert Griesemer:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from 张盛宇:

Patch Set 1:

(1 comment)

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Message from Ian Lance Taylor:

Patch Set 1:

(1 comment)

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