Bump version to 0.4.6 and add change log

[killercup]

No description provided.

[killercup]

Hm, 2 tests fail. Probably because of changes in the diagnostic output. I'll into those separately and rebase this later.

[ehuss]

Let me know if you need help with them.

Also, can this line be removed? It doesn't look like it really needs nightly, and that's the only thing requiring it. Probably doesn't matter much if you are guaranteeing the tests only pass on nightly, though, I was just surprised (and the error was not clear). Might also be good to have a note somewhere (docs or an assert in code) about the required rustc version, so people like me don't get confused by errors using the wrong version.

[ehuss]

I posted #172 to update the tests.