replace []Issue with []*Issue and for other large structs as well

[gmlewis]

Note that this is an API-breaking change but should have minimal
impact on users of this package due to the nice inference
properties of the Go programming language.

Bumped libraryVersion to 2 due to API-breaking change as
discussed in #376.

Fixes #180.

Change-Id: Ib386135e6b8f306d1f54278968c576f3ceccc4e7

[dmitshur]

Most clients will not need to change their code at all. Some will need to update it, if they have wrappers or mock implementations of these interfaces. But it's easy to do so, and it makes the API better/resolves an issue.

It is a breaking API change (even if slightly), so I'd suggest waiting to hear @willnorris's thoughts too. But I think it's worth it (at least I'm not opposed).

LGTM.

P.S. You would not be able to do this in C++ as easily, because each . in user code would have to become ->. :)

[willnorris]

I think more people are going to have to change their code than you think. For example, just looking at the Issues.List method, both of the two people using it would have compilation errors. Now granted, it's a very easy fix for them, but it's still a breaking change nonetheless.

That said, I'm still fine with doing it.