growth of mutable lists

@DanGrayson is already working on this.

Since Jul 27, 2017.

It looks like mutable lists grow their length by only one at a time. Perhaps
the length should double when it needs to increase. Or just warn users to
amortize the cost themselves.
```
i36 : x = new MutableList

o36 = MutableList{}

o36 : MutableList

i37 : x#4=1

o37 = 1

i38 : peek x

o38 = MutableList{null, null, null, null, 1}

i39 : x#5=1

o39 = 1

i40 : peek x

o40 = MutableList{null, null, null, null, 1, 1}
```

from evaluate.d:
```
assignvector(m:List,i:Code,rhs:Code):Expr := (
x := m.v;
ival := eval(i);
when ival
is j:ZZcell do (
if isInt(j)
then (
k := toInt(j);
if k < 0 then k = k + length(x);
if k < 0 then return printErrorMessageE(i,"negative subscript out of bounds 0 .. "+tostring(length(x)-1));
val := eval(rhs);
when val is Error do return val else (
if k >= length(x) then (
x = new Sequence len k+1 do (
foreach t in x do provide t;
while true do provide nullE;
);
m.v = x; );
x.k = val;
val))
else printErrorMessageE(i,"expected small integer"))
is Error do ival
else printErrorMessageE(i,"index not an integer")
);
```