Add "new MutableList from ZZ"

[d-torrance]

We add a new constructor for MutableList suggested in an old bugs/dan file:

i1 : new MutableList from 6

o1 = MutableList{...6...}

o1 : MutableList

i2 : peek oo

o2 = MutableList{null, null, null, null, null, null}

We also improve the MutableList docs a bit, mentioning a couple things suggested in another old bugs/dan file.

[mahrud]

I would prefer a different notation here. Maybe something involving allocate or size rather than squeezing into NewFromMethod, which is meant to be for subclasses only.