Allow Decimal's constructor to take a Number or String.

[zslayton]

See this discussion.

Constructing a dom.Decimal (which is typically what users want) from a number or string cannot be done in a single step unless you use ion.load(...), which is overkill.

Value.from(...) doesn't work. Because there is no hinting system (#620), Value.from will never return a dom.Decimal; a String maps to a dom.String, an integer Number maps to a dom.Integer, and a non-integer Number maps to a dom.Float.

[desaikd]

Fixed with #746