rust-dependencies: upgrade `micro-timer` dependency

I wanted to to a tour of dependencies to upgrade, but only `micro-timer` has a
new release which does not print when the function panics, which should be less
misleading.

Differential Revision: https://phab.mercurial-scm.org/D8605

rust/hg-core/Cargo.toml

@@ -21,7 +21,7 b' regex = "1.3.9"'
 twox-hash = "1.5.0"
 same-file = "1.0.6"
 crossbeam = "0.7.3"
-micro-timer = "0.2.1"
+micro-timer = "0.3.0"
 log = "0.4.8"
 
 [dev-dependencies]