Major revamp of shell init - drop bash support, focus on zsh

zsh/rust.zsh

@@ -0,0 +1,15 @@
+#
+# Rust environment setup.
+#
+
+# Rustup
+if [ -d "$HOME/.cargo/bin" ]; then
+  path_prepend "$HOME/.cargo/bin"
+fi
+
+install_rust_global_packages() {
+  rustup component add rustfmt-preview
+  rustup component add rust-src
+  cargo +nightly install --force clippy
+  cargo install --force racer
+}