Handle code blocks annotated with `// BUILD TIME` by generating a
`build.rs` instead of `src/main.rs`, adding `mingling` as a build
dependency with the `builds` feature, and writing a stub main.rs

Add `mingling_pathf` sub-crate and `pathf` feature for automatic
resolution of Mingling type module paths at build time. Scans source
files, identifies macro invocations via pattern matchers, and generates
mapping files consumed by `gen_program!()`.