hashmap: schema changes to root node, tuple nodes, use of 'Any'

[rvagg]

BREAKING change to the block representation in two ways:

• nested root node, now within hamt property (I can't recall who suggested this but the name is a nice clue about what the block contains if you're looking at the map at the root).
• tuple representation for nodes

Also (cosmetic):

• use of 'Any' instead of custom 'Value' that does the same thing

[rvagg]

note that the first node is inline, so they look like this:

root:

{
  "hashAlg": 18,
  "bucketSize": 3,
  "hamt": [
    <bytes>,
    [ ... data ... ]
  ]
}

whereas intermediates and leaves look like:

[
  <bytes>,
  [ ... data ... ]
]