Impl conversion from array to Vec

[rjsberry]

This PR implements conversion from arrays to vec. The array does not have to be the same length as the capacity of the vec. Const generics are asserted at compile time. Unfortunately the method currently requires a copy of the source array.

Partially addresses requests in #341 and #344.

Also I'm not totally happy about the const assertions given #351. I've not added any tests in the cfail suite as I don't think we can practically check this with trybuild at the moment. There is a bit of a work-around you can do but it's not pretty.

Questions

1. Would we prefer to just implement From<[T; N]> for Vec<T, N>? One of the issues with this impl is that users will need to be explicit about vec capacity. Because lengths N and M can be different, you can't write:

   let v = Vec::from([1, 2, 3]);

   It needs to be:

   let v = Vec::<_, 3>::from([1, 2, 3]);

2. Should from_array be internal, and the API exposed by the crate simply be the From trait?

[Dirbaio]

thank you!