Takes an input map and transforms it according to the rules
described in xform.  xform is a map of keys :or, :select, :delete,
:update, :rename and :project.  The transformations are applied
specifically in this order -- keys are maybe added, selected,
deleted, updated and renamed -- and all are optional.  :project is a
convenience that combines the behaviour of :select and :rename; it
selects the keys from the map and then renames as :rename does.  If
both :select and :project are specified, the values for :select and
keys from :project are both selected (but the :project keys then get
renamed, of course).

:assoc   - A map of key/value pairs to overwrite in the map.
:or      - A map of key/value pairs to add to the map if the keys are
           missing.
:select  - A seq of keys to select from the input map.
:delete  - A seq of keys to delete from the input.
:update  - A map from keys in the input map to functions to apply to
           the corresponding values using update.
:rename  - A map from old keys to new keys.
:project - A map that combines the behaviour of :select and :rename
           by selecting the keys of the map and then treats it as a
           rename operation.