Add a generic implementation of quadratic hash table probing.

We have multiple pre-computed constant hash tables that all use the same
quadratic probing algorithm.

Add a constant_hash Rust module to match the meta/constant_hash.py module.

Move the simple_hash() function into constant_hash. Its Python equivalent is in
the constant_hash.py module.

src/libcretonne/settings.rs

@@ -23,7 +23,7 @@
 use std::fmt;
 use std::result;
 
-use simple_hash::simple_hash;
+use constant_hash::simple_hash;
 
 /// A string-based configurator for settings groups.
 ///