- Update ValidFunctionNameSniff to flag underscore-prefixed protected/private
  methods (except Entity accessor/mutator pattern _get*, _set*)
- Add ValidPropertyNameSniff to flag underscore-prefixed properties
- Remove PSR2.Classes.PropertyDeclaration.Underscore exclusion from ruleset
- Add new Slevomat sniffs:
  - RequireNullCoalesceEqualOperator
  - RequireNullSafeObjectOperator
  - RequireSelfReference
  - NullTypeHintOnLastPosition
- Fix $_magicMethods property naming in sniff itself
- Add T_ENUM support to ValidFunctionNameSniff

Relax the ValidEnumNameSniff to allow omitting the "Enum" suffix when
the enum is in a namespace containing "Enum" as a segment (e.g.,
`App\Model\Enum\Status` is now valid without the suffix).

This avoids redundant naming like `App\Model\Enum\FooBarEnum`.

Refs #425

Fix conflict between TypeHintSniff and Slevomat null position sniffs

Partial namespace references like Mockery\MockInterface were not being
flagged by ReferenceUsedNamesOnly because Slevomat's allowPartialUses
defaults to true. Set it to false so such references are required to be
imported via a use statement.