2.4.1078

• Address #507 by clarifying formatting of :cast in Special Syntax.
• Fix #505 by rewriting the helper merge function to handle both keywords and symbols properly.
• Address #503 by adding :at-time-zone special syntax.
• Address #504 for BigQuery support, by adding special syntax for ignore/respect nulls, as well as new :distinct and :expr clauses to allow expressions to be qualified with SQL clauses. The latter will probably be useful for other dialects too.
• Update tools.build to 0.9.6 (and get rid of template/pom.xml in favor of new :pom-data option to b/write-pom).