[activemodel] Implement ActiveModel$Base methods

[yukihirop]

Summary

The remaining methods to be implemented are as follows

- [ ] MODULE ActiveModel::AttributeAssignment
- [x] MODULE ActiveModel::AttributeMethods
- [ ] MODULE ActiveModel::Attributes
- [x] MODULE ActiveModel::Callbacks
- [x] MODULE ActiveModel::Conversion
- [x] MODULE ActiveModel::Dirty
- [ ] MODULE ActiveModel::Lint
- [x] MODULE ActiveModel::Model
- [x] MODULE ActiveModel::Naming
- [ ] MODULE ActiveModel::SecurePassword
- [ ] MODULE ActiveModel::Serialization
- [ ] MODULE ActiveModel::Serializers
- [x] MODULE ActiveModel::Translation
- [ ] MODULE ActiveModel::Type
- [ ] MODULE ActiveModel::VERSION
- [x] MODULE ActiveModel::Validations
- [ ] CLASS ActiveModel::DeprecationHandlingDetailsHash
- [ ] CLASS ActiveModel::DeprecationHandlingMessageArray
- [ ] CLASS ActiveModel::DeprecationHandlingMessageHash
- [ ] CLASS ActiveModel::Error
- [ ] CLASS ActiveModel::Errors
- [ ] CLASS ActiveModel::ForbiddenAttributesError
- [ ] CLASS ActiveModel::MissingAttributeError
- [ ] CLASS ActiveModel::Name
- [ ] CLASS ActiveModel::NestedError
- [ ] CLASS ActiveModel::RangeError
- [ ] CLASS ActiveModel::StrictValidationFailed
- [ ] CLASS ActiveModel::UnknownAttributeError
- [ ] CLASS ActiveModel::ValidationError
- [ ] CLASS ActiveModel::Validator