Adds basic support for class functions and methods. (#1057)

* adding methods to the ast

* pre commit stuff?

* implementation of class functions

* implemented methods

* some cleanup

* more cleanup

* add newlines in test programs

* pre-commit fixups

* added include of return_term.h

* a test of a method calling another method

* replacing Member with Declaration

* removing the member.h etc files

* clarify a type annotation

* update uses of FunctionDeclaration

* remove ReturnTarget, no longer needed

* more cleanup

* more cleanup

* yet more cleanup, playing with pre-commit

* did a pre-commit run --all-files

* fixed const issue

* remove comment

* checking dependencies in BUILD files and headers

* pre-commit working now

* refactor NominalClassType to just hold a pointer to the class declaration

* remove Member from rtti

* responding to Geoffreys review

* change field_types to a non-member function

executable_semantics/ast/ast_rtti.txt

@@ -50,5 +50,3 @@ abstract class Expression : AstNode;
   class IdentifierExpression : Expression;
   class IntrinsicExpression : Expression;
   class UnimplementedExpression : Expression;
-abstract class Member : AstNode;
-  class FieldMember : Member;