Casting of pointers in JS backend

[dom96]

I have made an attempt to get the marshal module working in the JS backend and it's not so easy. After fixing the lack of copyMem in streams.nim I was faced with a bunch of internal errors.

To fix the copyMem I added the following to streams.nim:

when defined(js):
  proc copyMem(dest, source: pointer, size: Natural) =
    for i in 0 .. <size:
      {.emit: """
        `dest`[`i`] = `source`[`i`];
      """.}

Then I started seeing internal errors in genDeref, on the following line in the typeinfo module:

...
result = cast[ptr bool](x.value)[]
...

Some relevant discussion on IRC is available here.

As far as @Araq told me, cast[T](x) is mapped to x in the JS backend and x[] is mapped to x[0], it therefore stands to reason that cast[T](x)[] should be mapped to x[0].

Not sure how to implement this unfortunately. There is also a problem when assigning a pointer to another pointer (an internal error in genAsgn). These errors can be mitigated to some extent by hacking around with emit but I don't think this solution is correct.