Input validator centered around Type::Tiny

Changes for 2.18

• fix: field's addons (internal) are no longer assignable by the constructor
• fix: FieldDefinitionBuilder's "data" is now named "build_data" not to clash with FieldDefinition's "data"
• simplify and optimize Plugin::Strict
• improved subform behavior with default and adjust options: see Manual::Compatibility (gh#11)
• documentation improvements and fixes (gh#9)