Clean up several minor bugbears in elixir-smie.

The changes are as follows:

1. Removed `COMMA` from the list of block intro keywords. This was the source of a whole class of problems.

2. Removed the unused second parameter from the signature for `elixir-smie-next-token-no-lookaround`.

3. `elixir-smie-grammar` is now a constant.

4. Changed code layout of `elixir-smie-grammar`.

5. Removed several `elixir-smie-rules` that were rendered redundant with changes in #74.

6. Removed a test that was testing deprecated syntax.