[sv-bc] Erratum and proposal in casting BNF

I think that the following syntactic possibilities in the
current BNF don't make semantic sense

     signed'{...}, unsigned'{...}, 13'{...}

and that the original intent was for {}-style casts to
always use an actual type, not one of the type modifiers.

Attached is a proposal that would bring the BNF into alignment
with that intent.

-- Brad