Copying tainted numbers does not propagate taint

[leeN]

The following piece of code works as expected:

let t = Number.tainted(42);
typeof t // number
t.taint // returns taint object
let t2 = Number(t);
typeof t2 // number
t2.taint // returns same taint object

However, if we do the following:

t = Number.tainted(42);
typeof t // number
t.taint // returns taint object
let t3 = Number(t);
typeof t3 // object
t3.taint // returns null

This does not seem like the desired behavior and might be the root cause for #218.