Change Union types sort order

[quinnj]

The order of types in a Union should be mostly irrelevant, but with WIP work on inline, isbits Union types, the "default initialization" for Arrays requires the underlying Union selector byte to be zeroed out (otherwise an array element could be tagged as an "out of bounds" type for the Union). There are a few conveniences if the Union types sort order followed a pattern like:

1. Singleton types
2. Sizeof isbits types
3. All reference types....alphabetically?

[nalimilan]

For a bit of context, it would be convenient if Union{T, Null} and Union{Null, T} had identical coding of their type tags, with 0 denoting Null. This is the convention used by Pandas, PostgreSQL, SQLite and Arrow/Feather. See also #18507. So sorting singleton types first would handle this case.

[quinnj]

Do we already have code that sorts Union types? Or is that just enforced in the show method for Unions? I'm happy to take a crack at this if someone could give a few tips or places to look/start.

[vtjnash]

No, show just sorts them alphabetically while printing them

[JeffBezanson]

Should probably be done in jl_type_union in jltypes.c, which handles normalization (currently, removing types subsumed by others in a union).

[quinnj]

Yeah, I'm stepping through that function right now actually. Is it ok if I just do a separate sort call after normalization but before the Union structs are made?

[JeffBezanson]

Yes that sounds fine.