Replace SliceConcatExt trait with inherent methods and SliceConcat helper trait

Before this change `SliceConcatExt` was an unstable extension trait
with stable methods. It was in the libstd prelude, so that its methods
could be used on the stable channel.

This replaces it with inherent methods,
which can be used without any addition to the prelude.
Since the methods are stable and very generic
(with for example a return type that depends on the types of parameters),
an helper trait is still needed.
But now that trait does not need to be in scope for the methods to be used.

Removing this depedency on the libstd prelude allows the methods to be used
in `#![no_std]` crate that use liballoc, which does not have its own
implicitly-imported prelude.

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