Constructs a object implementing the
function O.refdict(valueConstructorFn, sizeHint)
Both arguments are optional.
valueConstructorFn is a function, it will be called to construct a value for any unknown key. Set to
undefined if you don’t want to use a value constructor function, and are passing in a
sizeHint is passed, it will be used as a hint for the expected number of values in this dictionary. This is an optimisation only, and avoids reallocation. Only set it if you’re pretty much sure of how many objects you’ll be storing.