[dslx] Allow full grammar for expressions to be evaluated within parametric signatures

Current expressions that can appear embedded directly within the types in a parametric function are quite limited, as reflected in #967 -- it is possible to support the full expression grammar in those contexts, and it would be simplifying vs having a subset AST for parametric expressions as we do today. This issue is to track that enhancement, while #967 is used to fix those specific internal errors (which should not occur for any user input).