Fix building under current Rust master (std::vec dep).

[tekacs]

Changes are just:

use std::vec;
...
Vec -> vec::Vec

Do switch this out if there's a tidier way of doing this that I've ignored. The persistent overlap between Vec/Vec is the source of the verbosity.

[tekacs]

... aaand the tests also needed re-writing. Fixing.

[tekacs]

And now the last fix I have to leave to someone who knows what they're doing in this context, whilst I go and bring my Rust knowledge up to the current language version. :)

Presumably the trait for as_slice (via std::str::Str) needs to be brought into the current scope without overlapping with super::Str.