Cross-platform (clj/cljs) destructuring. Adapted from clojurescript codebase

A mount-point instance maintains :
* a hash map storing items indexed by tag. An item is a mutable object tracking the lifecycle of each entry in the
  resulting incseq.
* a set of active readers. Each mutation of the store sends an invalidation event for the item being touched to the
  readers currently active.

A reader process maintains :
* a call tree isomorphic to the subset of the application's call tree restricted to the ancestors of active items.
  Leaves are items, nodes are either blocks or calls, block children are items or calls, call children are blocks, the
  root is a call.
* a hash map storing blocks indexed by frame. A block is a mutable object tracking each known frame. A frame is known
  when either it's an ancestor of an active item, or it's currently being mounted by a call that is in the common
  ancestry of an active item.
* a mailbox for step events on calls and another one for invalidation events on items. Both mailboxes are consumed
  during reader process transfer, call events take priority over the item events but ordering of events within a single
  mailbox is irrelevant. The processing of each event mutates the call tree and generates a diff, then the concatenation
  of successive diffs is returned. When the reader is spawned, an invalidation event is posted for each active item.

On item invalidation event :
* If the item is inactive :
  * If it was attached in the call tree, it is detached from the tree and a shrink is generated if the item was mounted.
  * If it was detached from the call tree, nothing happens.
* If the item is active :
  * If it was attached in the call tree, a change is generated if the item was mounted.
  * If it was detached from the call tree, it is attached to the tree and a grow is generated if the item was mounted.

On call step event :
1. Apply permutation. The call permutation must be expanded to take into account the offset and length of the call
   segment in the current sequence state.
2. Apply changes. For each item change that is not a grow, the block associated to previous frame is unmounted. The new
   frame is then associated to its block and mounted.
3. Apply shrinks. The blocks associated with removed frames are unmounted.

Unmounting a block generates a shrink for each active item having this block's frame as an ancestor.
Mounting a block generates a grow for each active item having this block's frame as an ancestor.