deprecate method values for nullary methods

[adriaanm]

Currently, http://www.scala-lang.org/files/archive/spec/2.12/06-expressions.html#method-values says:

The expression e _ is well-formed if e is of method type or if e is a call-by-name parameter. [...]
If e is a parameterless method or call-by-name parameter of type => T, e _ represents the function of type () => T, which evaluates e when it is applied to the empty parameter list ().

Since we've removed eta-expansion for nullary methods, for consistency we should also deprecate and drop the last part re: "parameterless method or call-by-name parameter". Those will need to be written explicitly using () => e.

Eventually (ideally), all method references will be eta-expanded regardless of expected type, and method value syntax will be dropped since it's redundant.

[adriaanm]

Duplicate of #273