ast/simplify: improve enum handling.

ast/simplify: improve enum handling.

Before this commit, enum values were serialized as attributes of form
  \enum_<width>_<value>
where <value> was a decimal signed integer.

This has multiple drawbacks:
  * Enums with large values would be hard to process for downstream
    tooling that cannot parse arbitrary precision decimals. (In fact
    Yosys also did not correctly process enums with large values,
    and would overflow `int`.)
  * Enum value attributes were not confined to their own namespace,
    making it harder for downstream tooling to enumerate all such
    attributes, as opposed to looking up any specific value.
  * Enum values could not include x or z, which are explicitly
    permitted in the SystemVerilog standard.

After this commit, enum values are serialized as attributes of form
  \enum_value_<value>
where <value> is a bit sequence of the appropriate width.