Solo is the canonical 1-element tuple type.
However, the implementation added in PR #891 treat it as any other normal 1-element type, which is incorrect.

This becomes problematic when you have code that wants to seriaize/deserialize generically over tuples of various arities:

• (10, 20, 30) <-> "[10, 20, 30]"
• (10, 20) <-> "[10, 20]"
• Solo 10 <-> "10" ❗❗❗
• () <-> "[]"