Redefine try macro to omit From::from error conversion

[dtolnay]

None of the error handling in serde needs the From::from error conversion performed implicitly by the ? operator or the standard library's try! macro. This PR adds a simplified macro that gives a 5.5% improvement in compile time compared to standard try!, and 9% improvement compared to ?. The same trick has already been in use by serde_derive since #1226.

	check time	rmeta size	build time	rlib size
standard library try!	2.15 sec	3,568 K	2.53 sec	6,943 K
? operator	2.23 sec	3,568 K	2.62 sec	7,137 K
this PR	2.03 sec	3,569 K	2.39 sec	6,829 K

build time is measured by time rustc --cfg 'feature="std"' serde/src/lib.rs --crate-type=lib, and check time is the same but with --emit=metadata.