Move from "nightly" toolchain to "stable".

Intended to address tau RefID "Gifn1u".

There are still a few things that require nightly.
- cargo +nightly fmt
- fuzzing

bin/darkirc/Makefile

@@ -4,7 +4,7 @@
 PREFIX = $(HOME)/.cargo
 
 # Cargo binary
-CARGO = cargo +nightly
+CARGO = cargo
 
 # Compile target
 RUST_TARGET = $(shell rustc -Vv | grep '^host: ' | cut -d' ' -f2)