lambda syntax with no captures should make an `extern fn`, not `&'static fn`

[thestinger]

It's silly to output the closure header and use wide pointer just for a bare function with no captures. An extern fn is implicitly coerced to &'static fn so this shouldn't cause any backwards compatibility issues.

[thestinger]

No longer possible without noticeably changing semantics. Unboxed closures would work fine for this anyway.