[ideas] providing generic functions to compare 'a against a Js.null

[bobzhang]

val eqlNull : 'a  -> box:'a Js.null -> bool 
val eqOpt : 'a -> box:'a option -> bool

note null =~ null should return true
polymorphic comparison warning should emitted when it can not be specalized

Use cases:

eqNull ~null:(Array.getNull a 0) 3 

otherwise, we have to check nullable and compare it later which is too verbose.

Do we need make use of nullary constructor?

eqOpt A ~box:v
eqNull A ~box:v

Note the option specialize is tricky:

• when the left operand value is A, it can not be specialized A = None
• when the left operand type is enum(not maybe_pointer), it still can not be specialized.
  it can be only specialized based on the type of first argument, given the complexity introduced, we won't do it