Change `for` protocol so that `for` functions return a bool

[nikomatsakis]

I argued in this blog post that this would be an improvement. Everyone seemed to agree. Nominating for milestone. I couldn't find an existing bug on this precise topic, but let me know if there is one.

[nikomatsakis]

In terms of the compiler, this is mostly a matter of modifying the type checker so that for statements have unit type, and modifying trans to ignore the return value.