Support for "non-standard" directive syntax: Inline crates

[fmease]

Examples:

//@ edition: 2021

fn main() {
    alpha::f();
    beta::g();
    h();
}

//@ crate alpha {
//@ edition: 2015
    pub fn f() {}
//@ }

//@ crate beta {
    pub fn g() {}
//@ }

fn h() {}

They can nest arbitrarily. We probably create tempfiles for inline crates (in /tmp/). However, if we ever add support for "--out-dir", we can also dump them there if applicable.

//@ crate crate_name "file_name" {
//@ }

//@ crate _ "test_name" {
//@ }

//@ raw-crate name {
//  Doesn't get added as a dependency of the "containing"/"super" crate.
//  Use `aux-build`, `aux-crate`, `compile-flags`, `proc-macro` for that.
//  Allows for more fine-grained control.
//@ }

[fmease]

Paving the way: 1d9951e