Add a PredicateView type to abstract the predicate bit vector a bit.

The encoding tables contain references to numbered ISA predicates.

- Give the ISA Flags types a predicate_view() method which returns a
  PredicateView.
- Delete the old predicate_bytes() method which returned a raw &[u8].
- Use a 'static lifetime for the encoding list slice in the Encodings
  iterator, and a single 'a lifetime for everything else.

lib/cretonne/src/settings.rs

@@ -162,6 +162,28 @@ pub enum Error {
 /// A result returned when changing a setting.
 pub type Result<T> = result::Result<T, Error>;
 
+/// A reference to just the boolean predicates of a settings object.
+///
+/// The settings objects themselves are generated and appear in the `isa/*/settings.rs` modules.
+/// Each settings object provides a `predicate_view()` method that makes it possible to query
+/// ISA predicates by number.
+#[derive(Clone, Copy)]
+pub struct PredicateView<'a>(&'a [u8]);
+
+impl<'a> PredicateView<'a> {
+    /// Create a new view of a precomputed predicate vector.
+    ///
+    /// See the `predicate_view()` method on the various `Flags` types defined for each ISA.
+    pub fn new(bits: &'a [u8]) -> PredicateView {
+        PredicateView(bits)
+    }
+
+    /// Check a numbered predicate.
+    pub fn test(self, p: usize) -> bool {
+        self.0[p / 8] & (1 << (p % 8)) != 0
+    }
+}
+
 /// Implementation details for generated code.
 ///
 /// This module holds definitions that need to be public so the can be instantiated by generated