Release version 0.9.4 (#184)

There have been a lot of changes since the 0.9.3 release, and it looks
like they improve the quality of generated code in cranelift. Let's
release 0.9.4!

Author: elliottt

Cargo.toml

@@ -3,7 +3,7 @@ members = ["regalloc2-tool"]
 
 [package]
 name = "regalloc2"
-version = "0.9.3"
+version = "0.9.4"
 authors = [
     "Chris Fallin <[email protected]>",
     "Mozilla SpiderMonkey Developers",