Remove fn expressions

[nikomatsakis]

Expressions like fn(S) { ... } are deprecated in favor of |s| t. I have a patch removing them and also implemented #3320 (better inference) but it is blocked on #2263.

[ben0x539]

fn expressions currently seem to permit return, allowing for gross little things like short-circuiting out of a loop yielding a value without declaring a proper, non-anonymous function. Is that use deprecated too?

[nikomatsakis]

Presumably. I could imagine loosening the "no return in ||" rule to something like "no return when using the do keyword", or perhaps even looser like "no return when using the do keyword and a stack closure". @pcwalton has complained about this restriction annoying him; it's saved me a few times, though, when I threw in a "do foo { ... }" around a block of code that contained a return (and which would therefore no longer have returned as I expected).

[pcwalton]

Done.