typeck: boxed closures can't capture by value

closes #19141
closes #20193
closes #20228
auto merge of #20244 : japaric/rust/bc-no-move, r=nikomatsakis

closes #19141
closes #20193
closes #20228

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Currently whenever we encounter `let f = || {/* */}`, we *always* type check the RHS as a *boxed* closure. This is wrong when the RHS is `move || {/* */}` (because boxed closures can't capture by value) and generates all sort of badness during trans (see issues above). What we *should* do is always type check `move || {/* */}` as an *unboxed* closure, but ~~I *think* (haven't tried)~~ (2) this is not feasible right now because we have a limited form of kind (`Fn` vs `FnMut` vs `FnOnce`) inference that only works when there is an expected type (1).

In this PR, I've chosen to generate a type error whenever `let f = move || {/* */}` is encountered. The error asks the user to annotate the kind of the unboxed closure (e.g. `move |:| {/* */}`). Once annotated, the compiler will type check the RHS as an unboxed closure which is what the user wants.

r? @nikomatsakis 

(1) AIUI it only triggers in this scenario:

``` rust
fn is_uc<F>(_: F) where F: FnOnce() {}

fn main() {
    is_uc(|| {});  // type checked as unboxed closure with kind `FnOnce`
}
```

(2) I checked, and it's not possible because `check_unboxed_closure` expects a `kind` argument, but we can't supply that argument in this case (i.e. `let f = || {}`, what's the kind?). We could force the `FnOnce` kind in that case, but that's ad hoc. We should try to infer the kind depending on how the closure is used afterwards, but there is no inference mechanism to do that (at least, not right now).
syntax: format: remove unused method_statics field.
syntax: format: put static arrays in their own blocks to avoid needing a wrapper block.
syntax: turn the match-call generated by format_args inside-out.
syntax: use std::string::String unqualified in format.
syntax: change format_args! to produce fmt::Arguments instead of calling a function with them.
Fallout of changing format_args!(f, args) to f(format_args!(args)).