Specify derives for generated collection

[dmfutcher]

It would often be useful for the generated collection to have derives such as Debug or Clone. However, this is moderately complicated by the fact that not all item types are guaranteed to have Debug etc.

Defaulting to shadowing the Item type's derives (obviously minus WrappedVec!) and having a parameter to override with a specific list of traits to derive should be a good solution.

[dmfutcher]

Per rust-lang/rust#40574, there's no way for WrappedVec proc macro to see what other traits the item type is deriving, so having to manually specify them seems the best way forward, if not the most ergonomic solution. It may be possible to come up with a reasonable default set of derivable traits but that could make the situation even more confusing (compiler error messages are pretty bad).

[dmfutcher]

Implemented in v0.2.1