Add a settings::Builder data type.

- Move detail data structures into a settings::detail module to avoid polluting
  the settings namespace.

- Rename generated data types to 'Flags' in anticipation of computed predicate
  flags that can't be set. The Flags struct is immutable.

- Use a settings::Builder struct to manipulate settings, then pass it to
  Flags::new().

src/libcretonne/isa/riscv/settings.rs

@@ -1,20 +1,21 @@
 //! RISC-V Settings.
 
-use settings::{Descriptor, Detail, Stringwise, Result, Error};
+use settings::{detail, Builder};
 use std::fmt;
 
-// Include code generated by `meta/gen_settings.py`. This file contains a public `Settings` struct
+// Include code generated by `meta/gen_settings.py`. This file contains a public `Flags` struct
 // with an impl for all of the settings defined in `meta/cretonne/settings.py`.
 include!(concat!(env!("OUT_DIR"), "/settings-riscv.rs"));
 
 #[cfg(test)]
 mod tests {
-    use super::Settings;
+    use super::{builder, Flags};
 
     #[test]
     fn display_default() {
-        let s = Settings::default();
-        assert_eq!(s.to_string(),
+        let b = builder();
+        let f = Flags::new(b);
+        assert_eq!(f.to_string(),
                    "[riscv]\n\
                     supports_m = false\n\
                     supports_a = false\n\