Previously, an empty argument was treated as "not an argument at all". Now,
every function call always has at least one argument -- i.e., %foo{} is a
function call whose only argument is "" -- and %foo{,bar} is valid syntax.
This was causing a problem with situation where }} would have semantic meaning
other than escaping a }. Specifically, %func{%func{arg}} contains a }} but
should not escape the }. $} seems to cover this situation. However, ${ is not
permitted as an escape sequence because it looks like the beginning of a symbol
(variable reference) like ${foo}. This is OK because { can be used anywhere as a
literal.
