support dict functions

[fiji-flo]

• set: Takes a dict, a key, and a value, and sets that key/value pair in
  the dict. set $dict $key $value. For convenience, it returns the dict,
  even though the dict was modified in place.
• unset: Takes a dict and a key, and deletes that key/value pair from the
  dict. unset $dict $key. This returns the dict for convenience.
• hasKey: Takes a dict and a key, and returns boolean true if the key is in
  the dict.
• pluck: Given a key and one or more maps, get all of the values for that key.
• keys: Get an array of all of the keys in a dict.
• pick: Select just the given keys out of the dict, and return a new dict.
• omit: Return a dict without the given keys.