arc4.Tuple with mutable element changes fails compilation

[achidlow]

Although tuples are immutable collections, their individual items can change if they are mutable.

Ideally, the following should compile and execute correctly

t = arc4.Tuple((arc4.UInt64(1), arc4.DynamicBytes(b"abc")))
assert t[1].bytes[2:] == b"abc", "initial value"
t[1].extend(arc4.DynamicBytes(b"def"))
assert t[1].bytes[2:] == b"abcdef", "updated value"