support unary operators (BREAKING CHANGE)

<:value< current_timestamp >> was previously special-cased in the
grammar to expand to Sql.Op.current_timestamp, while other identifiers
were interpreted as row names. I removed this special case and added
a `unit` value in SQL-land, so you can now write

  <:value< current_timestamp () >>
Merge pull request #19 from ocsigen/unary_ops

support unary operators (BREAKING CHANGE)