Most of the way to no_std support

lib/cretonne/src/settings.rs

@@ -24,6 +24,7 @@ use constant_hash::{probe, simple_hash};
 use isa::TargetIsa;
 use std::fmt;
 use std::result;
+use std::vec::Vec;
 
 /// A string-based configurator for settings groups.
 ///