Minimum viable virtual methods

[vgrichina]

My understanding is that it's possible to implement virtual methods in AssemblyScript before GC/reftypes land in WASM:

• Make compiler emit vtables for classes not marked as @unmanaged
• Make __memory_allocate take another optional parameter (vtable pointer). This can be stored before returned memory address, so that code expecting same object field offsets as now doesn't break.
• Use call_indirect for method calls (looking up function index in vtable)