rust/chg/build.rs

upgrade: Use `improvement` subclasses everywhere, not instances

This changes the source definition of optimizations to match that of formats:
a subclass with a decorator, instead of an instance passed to a function call.
Not having any instance removes the confusion between class attributes and
instance attributes, which were used interchangeably.

Differential Revision: https://phab.mercurial-scm.org/D10768

fn main() {
    cc::Build::new()
        .warnings(true)
        .file("src/sendfds.c")
        .file("src/sighandlers.c")
        .compile("procutil");
}