Add settings::Stringwise.

This trait allows settings to be manipulated as strings, using descriptors and
constant hash-table lookups.

Amend gen_settings.py to generate the necessary constant tables.

src/libcretonne/isa/riscv/settings.rs

@@ -1,5 +1,24 @@
 //! RISC-V Settings.
 
+use settings::{Descriptor, Detail, Stringwise, Result, Error};
+use std::fmt;
+
 // Include code generated by `meta/gen_settings.py`. This file contains a public `Settings` 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;
+
+    #[test]
+    fn display_default() {
+        let s = Settings::default();
+        assert_eq!(s.to_string(),
+                   "[riscv]\n\
+                    supports_m = false\n\
+                    supports_a = false\n\
+                    supports_f = false\n\
+                    supports_d = false\n");
+    }
+}