Fast enum conversion

This crate provides zerocost conversion between enums. An enum Src can be converted to another enum Dest when the following satisfies:

• For each variants of Src, counterparts exist in Dest.

Here, an variant ans its "counterpart" satisfies all of them:

• Both has the same tag names
• Fields of both variants are equal. For example, one has structural fields, another should have the same.
• All corresponding type of Fields are the same.

It performs zerocost conversion when following satisfies for all counterparts:

• The [std::mem::Discriminant] of them are equal.
• The fields has same offsets.
• Have consistent memory layouts.