Refactor to reduce compilation time (#1284)

- Make `binstalk::bins` private.
 - Move mod `signal` into `crates/bin`
 - Make items in `crates/bin/src/lib.rs` private if possible to reduce
   its API generation time.

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

crates/binstalk/src/lib.rs

@@ -1,6 +1,6 @@
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 
-pub mod bins;
+mod bins;
 pub mod drivers;
 pub mod errors;
 pub mod fetchers;