Optimize compilation time (#320)

* Print `rerun-if-changed=build.rs` in `build.rs`
* Optimize compile-time: Extract `bin/src/lib.rs`
   
   so that `cargo-binstall (lib)` can be compiled in parallel to other
   deps.
* Refactor: Extract new mod `bin/src/bin_utils.rs`
* Extract new fn `MainExit::new`
* Refactor: Extract new fn `run_tokio_main`
* Handle `Runtime::new` err gracefully in `run_tokio_main`
   instead of `panic!`ing, return the error as `BinstallError`
* Avoid mixing `eprintln` and `error` in `MainExit::report`
* Set profile for `build-override`
   to speedup building of `build.rs` and proc macros.

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

Cargo.toml

@@ -14,3 +14,19 @@ lto = true
 codegen-units = 1
 panic = "abort"
 strip = "symbols"
+
+[profile.dev.build-override]
+opt-level = 0
+codegen-units = 1024
+debug = false
+debug-assertions = false
+overflow-checks = false
+incremental = false
+
+[profile.release.build-override]
+opt-level = 0
+codegen-units = 1024
+debug = false
+debug-assertions = false
+overflow-checks = false
+incremental = false