Binary encoding of string.new_wtf16_array and other array methods

[kripken]

I noticed when fuzzing Binaryen vs V8 that V8 disallows an input of (ref.null none) to string.new_wtf16_array. That seems like it should be valid since it is a subtype of the array types. Talking to @tlively , he pointed out that other specs avoid this issue by having a type annotation in the binary format, which this spec seems to lack atm.

(This is not urgent in any way, just filing this to remind us if/when activity on this spec resumes.)