Change `ConditionallySelectable` supertrait

[tarcieri]

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To resolve #94, removes the Copy supertrait bound on ConditionallySelectable, replacing it with Sized instead.

It turns out the bound is only used in the default implementation of ConditionallySelectable::conditional_swap, and is easy to replace by slightly changing that default implementation.

Removing this supertrait bound is arguably a breaking change since it means types which impl ConditionallySelectable can no longer be assumed to be Copy, so also bumps the version to 3.0.0-pre.

Alternative to #118, #136