compact encoding of union variants

[soc]

Make unions more space-efficient after https://github.com/core-lang/core/issues/8 is implemented, ideally avoiding the need for additional space to store bits that denote the variant of a union value.

For class-based unions:

• consider the lower bits of the reference itself
• figure out what to do with union variants that are themselves unions

For value-based unions:

• employ niche-oriented encoding
• investigate passing variant info out-of-band (status flags (Swift appears doing something like that), additional reg, ...)

For mixed unions:

• figure out what the actual rules and requirements are