Procedural macros for #[export], #[export_native], #[export_wstp],
#[export_wxf], and #[init].
Emitted paths are resolved dynamically at expansion time via
proc-macro-crate: if the caller's Cargo.toml has wolfram-export the
macro emits ::wolfram_export::*; if it has wolfram-library-link (legacy)
it emits ::wolfram_library_link::*. Both crates expose the same hidden
runtime surface so generated code resolves correctly in both cases.