Allow block for Data#to_h

[ksss]

Data#to_h accepts a block that returns key-value pairs, as documented:

distance.to_h { |name, val| [name.to_s, val.to_s] }
#=> {"amount"=>"10", "unit"=>"km"}

This adds the block overload, aligned with Struct#to_h, and a test for both forms.

🤖 Generated with Claude Code