Fix "Unable to identify an operator =$" problem that occurred when pgsql

Fix "Unable to identify an operator =$" problem that occurred when pgsql
expressions were written without spaces between operators and operands.
Problem was that something like "if new.f1=new.f2 then" would be translated
to "if $1=$2 then", and the Postgres lexer would tokenize that the wrong
way.  Fix is to emit spaces around $paramno constructs to ensure they are
seen as separate tokens.