#7 Indexing lists with Long

[Kevin Rogers]

We hacked MillScript (in MapAwareTools.java) to
support Number when indexing a List. Strictly speaking
this is wrong. We should only support indexing by
integral types i.e. Integer, Long, BigInteger, BigDecimal
(with no fractional part), and the other wrapper types
such as Short etc.

We should have a convertToInt function that is
responsible for enforing the check.