Print errors with context; make coercions a library

Works around a cargo bug where `cargo rustdoc` does not pass flags to
the rustdoc binary.

Author: jyn514

collector/benchmarks/coercions/src/main.rs renamed to collector/benchmarks/coercions/src/lib.rs

@@ -65573,5 +65573,3 @@ pub static MAPPING: [&'static str; 0xffff] = [
     "",
     "",
 ];
-
-fn main() { }

collector/src/bin/rustc-perf-collector/main.rs

@@ -383,7 +383,7 @@ fn main() {
     match main_result() {
         Ok(code) => process::exit(code),
         Err(err) => {
-            eprintln!("{}", err);
+            eprintln!("{:#}", err);
             process::exit(1);
         }
     }