Handle buffer references vs 'const'

Allow constructors to and from references to be constant folded. Section 4.3.3
says constructors whose arguments are all constant expressions must fold.

Disallow 'const' on buffer reference types. It is not a 'non-void transparent
basic data type' (it is not considered 'basic').

Handle buffer reference constants (which can be assigned to a non-const reference,
or can be further folded to another type of constant) by converting to
'constructor(uint64_t constant)' in addConversion.

Disallow == and != operators on reference types.