Major Description
The message attribute for dfdl:assert and dfdl:discriminator is allowed to be a DFDL expression, for example:
<dfdl:assert message="{ fn:concat('unknown case ', ../data1) }">...</dfdl:assert>
However, we currently treat the message as a string, and never evaluate the expression, even if it contains braces. To resolve this issue, we should create a new Evaluatable for the message attribute and pass that into the assert/discriminator parsers.