Rename `examples` directory to `spec/fixtures`

- I want to move away from having complete grammars for real languages
  (e.g. javascript, golang) in this repo. These languages take a long
  time to compile, and they now exist in their own repos
  (node-tree-sitter-javascript etc).
- I want to start testing more compiler edge cases through integration
  tests, so I want to put more small, weird grammars in here. That makes
  me not want to call the directory `examples`.