dexed types are backed by a single chunk of memory for
fast access, and are indexed from 0 as in C.  Dictionary types associate keys
with values. The difference between dictionaries and indexed types is that
dictionaries are not limited to integer keys. They are backed by a hash table
and also offer constant time lookup (and insertion for the mutable case).
Finally, the 'bytes' abstraction is any type that contains a sequence of bytes.
A 'bytes' value or byteseq associates integer keys (the indices) with in