Support ES6 Symbols

[chocolateboy]

Symbols are unique, so the implementation added in #23 doesn't work as expected e.g.:

$ replem object-hash:hash

> s1 = Symbol("foo")
Symbol(foo)

> s2 = Symbol("foo")
Symbol(foo)

> s1 === s2
false

> o = { [s1]: "s1", [s2]: "s2" }
> o[s1] === o[s2]
false

> hash(s1) === hash(s2)
true

The last test should return false because the values are as distinct as two different numbers.

[addaleax]

The last test should return false because the values are as distinct as two different numbers.

The only issue is: how? I don’t think this would be possible without a global list of symbols that have been seen, but you really want a hash function to not depend on mutable global state, so … hm. You might be right, but this might just not be possible to do.