Significant refactor intended to simplify the grammar

This is a significant refactor intended to simplify the grammar, and reduce the number of constructs used where the
differentiation is not as significant as it looked previously.

• Refactor: simplify model overall, especially members and features.
  • Refactor: combine by-value and by-reference member types.
    • Remove: rules member_by_value and role_by_value.
    • Rename: rule member_by_reference to member and role_by_reference to member_role.
    • Remove: keyword the ref from rules member and member_role.
    • Rename: rule identity_member to entity_identity.
  • Refactor: combine structured type groups.
    • Rename: rule structure_body to structured_body.
    • Rename: rule structure_group to member_group.
    • Move: common parts of structure and entity bodies into common _structured_body_inner rule.
  • Refactor: move feature from a definition to a member modifier.
    • Remove: rule feature_set_def, and update definition accordingly.
    • Add: optional keyword features into the rule _type_expression_to.
• Feature: add new keyword opaque to datatype definition.
• Style: Use := throughout for assignment by definition.
  • Add: silent rule _by_definition.
  • Alter rules value_variant, function_def, and constant_def to use _by_definition.
• Style: rename rule _property_member to _property_reference.

Full Changelog: v0.1.42...v0.2.0