avoid potentially expensive operations on prepare

This simple patch makes sure we can operate with a reference to the
string instead of being forced to transform it to a string, and makes
sure that the Arc doesn't have to be cloned (which can be expensive in
multi-core systems).

This doesn't really make a large difference in benchmarks, given how
expensive Parse::new() is.

core/lib.rs

@@ -255,10 +255,10 @@ pub struct Connection {
 }
 
 impl Connection {
-    pub fn prepare(self: &Rc<Connection>, sql: impl Into<String>) -> Result<Statement> {
-        let sql = sql.into();
+    pub fn prepare(self: &Rc<Connection>, sql: impl AsRef<str>) -> Result<Statement> {
+        let sql = sql.as_ref();
         trace!("Preparing: {}", sql);
-        let db = self.db.clone();
+        let db = &self.db;
         let syms: &SymbolTable = &db.syms.borrow();
         let mut parser = Parser::new(sql.as_bytes());
         let cmd = parser.next()?;