Fix 658 and compiler complaints (#661)

* Tried but failed to fix `cargo build` complaint about unused #[macro use]. See also 7a803a8dc1
* Compiler complaints be-gone
* Give sumtree tests method-tagged folder names so they don't overwrite each others' files
Fixes #658

pool/src/lib.rs

@@ -32,7 +32,8 @@ extern crate grin_keychain as keychain;
 extern crate grin_util as util;
 extern crate rand;
 extern crate serde;
-#[macro_use]
+#[allow(unused_imports)]
+#[macro_use] // Needed for Serialize/Deserialize. The compiler complaining here is a bug.
 extern crate serde_derive;
 #[macro_use]
 extern crate slog;