Allow cargo to compile with >1 core

This should significantly speed up debug and test builds + cargo check.
With rust-lang/rust#42682, cargo/rustc no longer
spawns lots and lots of workers even when called recursively. Still not
enabled by default in release mode:

https://internals.rust-lang.org/t/help-test-out-thinlto/6017

Author: jonhoo

Cargo.toml

@@ -83,12 +83,6 @@ default-features = false
 # for Mac OS X build
 # git = "https://github.com/ms705/timekeeper"
 
-[profile.dev]
-codegen-units = 1
-
-[profile.test]
-codegen-units = 1
-
 [profile.release]
 debug=true