remove recursive solver

[nikomatsakis]

It seems like it's time to get rid of Chalk's "recursive" solver and commit to the on-demand SLG solver. It's basically strictly better at this point.

The recursive solver code is found here:

https://github.com/rust-lang-nursery/chalk/tree/master/src/solve/recursive

We'll also want to remove this SolverChoice variant:

https://github.com/rust-lang-nursery/chalk/blob/82a8472ca9f6756d160b639dfc68f6a69076b716/src/solve/mod.rs#L183-L187

Which will entail then modifying the unit tests. They mostly run on both solvers, as controlled here:

https://github.com/rust-lang-nursery/chalk/blob/82a8472ca9f6756d160b639dfc68f6a69076b716/src/solve/test/mod.rs#L72

But sometimes the tests have distinct results for the recursive vs SLG solvers:

https://github.com/rust-lang-nursery/chalk/blob/82a8472ca9f6756d160b639dfc68f6a69076b716/src/solve/test/mod.rs#L932-L937

And of course we'll have to modify chalki, the command-line interpreter too. We can just remove this arm:

https://github.com/rust-lang-nursery/chalk/blob/82a8472ca9f6756d160b639dfc68f6a69076b716/src/bin/chalki.rs#L226-L228

though we may just want to remove the whole --solver option.

[nikomatsakis]

cc @rust-lang-nursery/wg-traits -- simple way to get acquainted with chalk code