Use #write_method consistently.
Go back to explicitly injecting parent_attribute_values,
rather than relying on the reader method to find them.
Refactor: rename Clamp::{Option,Parameter,Subcommand}.

  - Clamp::Parameter to Clamp::Parameter::Definition
  - Clamp::Attribute to Clamp::Attribute::Definition
  - Clamp::Subcommand to Clamp::Subcommand::Definition