This patch implements the intrinsic for ref.null.
In the process of implementing int_wasm_ref_null_func() and
int_wasm_ref_null_extern() intrinsics, it removes the redundant
HeapType.
This also causes the textual assembler syntax for ref.null to
change. Instead of receiving an argument: func or extern, the
instruction mnemonic is either ref.null_func or ref.null_extern,
without the need for a further operand.
It would be good to undo these unrelated formatting changes.