Add support for generics

[jjedelsky]

This pull adds support for generics. See tests for examples.

[Arnavion]

@Yamakaky error-chain doesn't support generic errorkind, right?

[jjedelsky]

That is right, it doesn't.

[Arnavion]

Right, so error_chain errorkinds would no longer be guaranteed to be able to hold derive_error_chain errorkinds as chainable links. (Not a blocker, but worth noting.)

[jjedelsky]

I thing it should be enough not to implement From<generic> for Error.

[Arnavion]

Thanks!

I made some tiny changes:

• Removed the import for std::collections::HashSet since I prefer to use fully-qualified paths.
• Changed the code that builds the set to use collect() instead of a for-loop.
• Moved the generic-matcher to its own match arm.
• Added a check to the generic-matcher that the type path has only one segment so that it doesn't match some_mod::T.
• Some comments.