Add null Type Derivation.

Instead of implementing null as a special value assignable to all Object types, derive the type of null so it's simply an Object pointer value. While the type derivation is somewhat non-trivial this makes verification and code generation much easier.
	src/Compiler/AST/ExpressionIdentifier.h
	src/Compiler/AST/ExpressionIdentifier.cpp

Add a new pass, ResolveNullTypes, to identify what type of pointer null is being assigned to. This requires the types of the surrounding context have been established.
Add support for back-traversing the Expression hierarchy so we have more context when resolving the type of null.
	src/Compiler/AST/Expression.h
	src/Compiler/AST/Expression.cpp

Participants in the ResolveNullTypes pass.
	src/Compiler/AST/Method.h
	src/Compiler/AST/Method.cpp
	src/Compiler/AST/Statement.h
	src/Compiler/AST/Statement.cpp
	src/Compiler/AST/StatementAssign.h
	src/Compiler/AST/StatementAssign.cpp
	src/Compiler/AST/StatementBlock.h
	src/Compiler/AST/StatementBlock.cpp
	src/Compiler/AST/StatementDefinition.h
	src/Compiler/AST/StatementDefinition.cpp
	src/Compiler/AST/StatementIf.h
	src/Compiler/AST/StatementIf.cpp.
	src/Compiler/AST/StatementReturn.h
	src/Compiler/AST/StatementReturn.cpp
	src/Compiler/AST/StatementWhile.h
	src/Compiler/AST/StatementWhile.cpp

Also participants in ResolveNullTypes and updated to set the Parent back-reference.
	src/Compiler/AST/ExpressionBinary.h
	src/Compiler/AST/ExpressionBinary.cpp
	src/Compiler/AST/ExpressionFunction.h
	src/Compiler/AST/ExpressionFunction.cpp
	src/Compiler/AST/ExpressionMember.h
	src/Compiler/AST/ExpressionMember.cpp
	src/Compiler/AST/ExpressionUnary.h
	src/Compiler/AST/ExpressionUnary.cpp

Remove the special NullName string, as the Object type name is now used instead.
	src/Compiler/AST/Class.cpp
	src/Compiler/AST/Type.h
	src/Compiler/AST/Type.cpp
	src/Compiler/AST/Verifier.cpp
	src/Compiler/Language/MiniJava/Context.cpp
	src/Compiler/Language/MiniJava/ContextBuilder.h
	src/Compiler/Language/MiniJava/ContextBuilder.cpp