Closing Parenthesis

Lex `)` as CLOSING_PARENTHESIS.
Opening Parenthesis

Lex `(` as OPENING_PARENTHESIS.
Empty parentheses

Parse `(`, any EOL, and `)` as `emptyParentheses`, which is a valid
`expression`.
Add Empty Parentheses to README
Add code example for Empty Parentheses in README
Land #49, Empty Parentheses
Update changelogs