Get basic parsing working, add some unit tests

script/fetch-test-fixtures.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+grammar_dir='fixtures/tree-sitter-rust'
+grammar_url='https://github.com/tree-sitter/tree-sitter-rust'
+
+if [ ! -d $grammar_dir ]; then
+  git clone $grammar_url $grammar_dir --depth=1
+fi
+
+(
+  cd $grammar_dir;
+  git fetch origin master --depth=1
+  git reset --hard origin/master;
+)