Use an u8 for the tag of enums with up to 255 constructors

[yorickpeterse]

Description

The tag of an enum is now a u16, regardless of how many constructors there are. Most enums will have less than 255 constructors such that we can use a u8 instead in those cases. This in turn can be beneficial when storing an enum into another type, as depending on the order/padding of fields this can conserve space. One example is the FutureState type (see also #806).

We should change the compiler such that tags are lowered to a u8 if the enum has at most 255 constructors.

Related work

No response