Kernel calls through a qualified path are not exec-threaded

[byeongjee]

A call inside a #[kernel] body gets __exec threaded through it (and
::<N, _> supplied) only when the path is a single segment starting with a
lowercase letter or _ — rewrite.rs:1236-1249:

let single = p.qself.is_none()
    && p.path.leading_colon.is_none()
    && p.path.segments.len() == 1;

So m::k(x) and Self::k(x) compile as ordinary Rust calls with no exec
threading, and a kernel can only be called by a bare name.

The restriction is there because the prelude re-exports math, reduce and
rng as modules, so math::exp(x) and friends have to stay ordinary calls.
Telling the two apart is not possible syntactically — the macro never inspects
types — so this needs a marker of some kind.