style: declare globals properly

By default, `typeset` defines variables locally unless in the main scope.
This is specially bad when using `omz plugin load`, which happens inside
a function, so the declared variables don't continue being defined when
the function finishes and the main scope reappears.

plugins/rust/rust.plugin.zsh

@@ -11,7 +11,7 @@ command mkdir -p "$ZSH_CACHE_DIR/completions"
 # bind it to `cargo`. Otherwise, compinit will have already done that
 if [[ ! -f "$ZSH_CACHE_DIR/completions/_cargo" ]]; then
   autoload -Uz _cargo
-  declare -A _comps
+  typeset -g -A _comps
   _comps[cargo]=_cargo
 fi
 
@@ -19,7 +19,7 @@ fi
 # bind it to `rustup`. Otherwise, compinit will have already done that
 if [[ ! -f "$ZSH_CACHE_DIR/completions/_rustup" ]]; then
   autoload -Uz _rustup
-  declare -A _comps
+  typeset -g -A _comps
   _comps[rustup]=_rustup
 fi