Reorganize: Extract Statement and its impl into its own module.

src/cache.rs

@@ -5,6 +5,7 @@ use std::ops::{Deref, DerefMut};
 use lru_cache::LruCache;
 use {Result, Connection, Statement};
 use raw_statement::RawStatement;
+use statement::StatementCrateImpl;
 
 impl Connection {
     /// Prepare a SQL statement for execution, returning a previously prepared (but