Clock lists should be type stable

[ltratt]

Some VMs will optimise things like:

x = [-1] * 10
for i in range(10):
  x[i] = 0.0

in a way that allocates a list of unboxed ints on line 1, and then reallocs the list on line 3 to be a list of unboxed floats. We should create an array whose members never change type.

[ltratt]

The Python case is now fixed in #380; the JS case remains to be done.

[vext01]

In terms of JS, we had hoped to use typed arrays, however:

• There's no unsigned 64-bit type in the API (or 64-bit integer for that matter)
• Support is not universal.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Typed_arrays

So we will use NaN instead.

[ltratt]

The JavaScript case was fixed by #382. Closing.