Add support for setting presets.

Fixes #11.

Presets are groups of settings and values applied at once. This is used
as a shorthand in test files, so for example "isa intel nehalem" enables
all of the CPUID bits that the Nehalem micro-architecture provides.

lib/cretonne/src/isa/mod.rs

@@ -108,8 +108,8 @@ impl settings::Configurable for Builder {
         self.setup.set(name, value)
     }
 
-    fn set_bool(&mut self, name: &str, value: bool) -> settings::Result<()> {
-        self.setup.set_bool(name, value)
+    fn enable(&mut self, name: &str) -> settings::Result<()> {
+        self.setup.enable(name)
     }
 }