added splice feature (rust issue #32310)

[Pycckue-Bnepeg]

Now splice is unstable feature
rust-lang/rust#32310

[gyscos]

Thanks for the pull request!

Unfortunately, adding this feature would prevent this library from building on stable, which is a problem.

But we're not using Vec::splice here, we're using the method from odd::vec::VecExt.

[Pycckue-Bnepeg]

Yeap, my bad, didn't see last commits.
Cursive doesn't work from cargo, that's problem (at least yesterday).

[gyscos]

Ah just understood this wasn't released yet.
Just published 0.5.1 which should be working better.