Distinguish between record structs, tuple structs, and unit structs.

[jswrenn]

Deflect's pretty printing does not distinguish between record structs/variants, tuple structs/variants and unit structs/variants. This leads to some exceptionally ugly rendering of Option, Result, etc. Can we do better?

There's nothing in the DWARF DIEs of these types that distinguishes them. This heuristic might be acceptable:

• no fields: unit-like
• fields are in the form __\d+: tuple-like
• otherwise: struct-like

[bjorn3]

I believe those heuristics is what GDB does.