binding: Make parse methods more convenient

* Rename parse_str to parse and make it polymorphic.
* Rename parse_utf8 to parse_with, since it is now the callback-based
  version of parse
* Add a parse_utf16 method analogous to parse
* Rename existing parse_utf16 method to parse_utf16_with

This brings in the changes from tree-sitter/rust-tree-sitter#5

cli/src/parse.rs

@@ -33,7 +33,7 @@ pub fn parse_file_at_path(
 
     let time = Instant::now();
     let tree = parser
-        .parse_utf8(&mut |byte, _| &source_code[byte..], None)
+        .parse(&source_code, None)
         .expect("Incompatible language version");
     let duration = time.elapsed();
     let duration_ms = duration.as_secs() * 1000 + duration.subsec_nanos() as u64 / 1000000;