Accept &str in Parse::parse_sql (#182)

It is more generic to accept a `&str` than a `String` in an API,
and avoids having to convert a string to a `String` when not
needed, avoiding a copy.

examples/cli.rs

@@ -45,7 +45,7 @@ fn main() {
         chars.next();
         chars.as_str()
     };
-    let parse_result = Parser::parse_sql(&*dialect, without_bom.to_owned());
+    let parse_result = Parser::parse_sql(&*dialect, without_bom);
     match parse_result {
         Ok(statements) => {
             println!(

examples/parse_select.rs

@@ -23,7 +23,7 @@ fn main() {
 
     let dialect = GenericDialect {};
 
-    let ast = Parser::parse_sql(&dialect, sql.to_string()).unwrap();
+    let ast = Parser::parse_sql(&dialect, sql).unwrap();
 
     println!("AST: {:?}", ast);
 }