Reduce the resulting binary size from 3.0 to 2.2 MiB (in v0.9.4).

--- a/Cargo.toml
+++ b/Cargo.toml
@@ -48,3 +48,6 @@
 [profile.release]
 lto = true
 debug = false
+codegen-units = 1
+opt-level = "s"
+panic = "abort"