Exapand the definitions of default, implicit, and validator.{gt,lt,gte,lte} to accept either a literal scalar (as they do now), or a dictionary containing "expr: string" and optionally "is_constexpr: bool"

If a literal scalar, non-numerics will be encapsulated as needed (current behavior).

if a dictionary containing 'expr', that expression will be used without encapsulation.  is_constexpr (default true) will also enforce that the expression provided must be evaluable as a constexpr, failing to do so will result in a compile error.