Preserve mutability in "select"

[d-torrance]

This fixes an old bug in the bugs directory. Previously, select didn't keep track of whether its input was mutable or not, even though it did keep track of the class. So we got immutable MutableList objects!

Before

i1 : L = new MutableList from {0, 1, 2}

o1 = MutableList{...3...}

o1 : MutableList

i2 : M = select(L, i -> true)

o2 = MutableList{...3...}

o2 : MutableList

i3 : M#0 = 4
stdio:3:5:(3): error: assignment attempted to element of immutable list

After

i3 : M#0 = 4

o3 = 4

i4 : peek M

o4 = MutableList{4, 1, 2}