Pg: Add escape_literal() and escape_identifier()

Didn't expect I'd ever need these, but they're useful for generating SQL
scripts.

FU/Pg.pm

@@ -123,6 +123,17 @@ Connection is dead or otherwise unusable.
 
 =back
 
+=item $conn->escape_literal($str)
+
+Return an escaped version of C<$str> suitable for use as a string literal in an
+SQL statement. You'll rarely need this, it's often better to pass data as bind
+parameters instead.
+
+=item $conn->escape_identifier($str)
+
+Return an escaped version of C<$str> suitable for use as an identifier (name of
+a table, column, function, etc) in an SQL statement.
+
 =item $conn->cache($enable)
 
 =item $conn->text_params($enable)