Introduce deep paths for bindings and make views work with propagators

[gordonbrander]

This PR introduces a set of interdependent changes for deep key paths on bindings, as well as the beginnings of a propagator implementation.

To implement {{deep.paths.in.bindings}} we needed to introduce the concept of a "mapped reactive cell". The simplest way to do this turned out to be to create the beginnings of a propagators implementation.

For more on propagators, see

• https://dspace.mit.edu/handle/1721.1/44215
• https://dspace.mit.edu/handle/1721.1/54635
• https://groups.csail.mit.edu/mac/users/gjs/propagators/
• https://www.orionreed.com/posts/scoped-propagators
• https://observablehq.com/@gozala/propagators

Additions:

• Introduce @commontools/common-propagator package which offers a basic propagator implementation
  • Some basic diamond problem fixes are included for non-transformed sources via a Lamport clock
    • We may do further work in a future PR to make this more robust, following Radul, “Propagation Networks: A Flexible and Expressive Substrate for Computation.” or scheduling techniques from signals implementations.
  • Supports mergeable datatypes via the Mergeable interface (any type that implements .merge().
    • All other data types are LWW
  • lift() function to propagator for 1-8 arguments
  • lens() to create a cell that bi-directionally reflects the lensed contents of another cell
  • key() to create a cell that bi-directionally reflects a subkey of another cell
• Support deep indexing in view bindings {{a.b.c}}
  • Dotted paths in view template strings {{a.b.c}} are parsed into array paths `['a', 'b', 'c'] in view objects.
  • Underneath this feature is a function called path() that allows for deep pathing into objects.
    • Any type can be deep-pathed.
    • If type is Keyable (exposes a .key() method), this will be used to descend into the structure.
    • Otherwise, property access will be used to descend into the structure.
    • We implement Keyable for Cell, returning a mapped cell that bi-directionally reflects the sub-path of the parent.

Changes:

• Move vnode, binding, section, parser into view.js. These types are all interdependent on each other. Better to have them in one module to avoid circular dependencies.
  • vnode.tag changed to vnode.name. This is more appropriate, since we use name for other view node types, and the name may not always be a tag (in the case of documentfragment).
• Reactive, Cancel, and some other supporting types have been moved into common propagator
  • Sendable moved into @commontoos/common-propagator/reactive.js
• Parse sections (but don't do anything with them yet)
  • We will implement sections in a followup PR.
• Throw parse error in more cases where template string is invalid
• html template holes are filled with names generated by tid() rather than cid().
  • It's a bit silly, but I wanted the template system to independently own its own client-ID generation.

Removed:

• Removed State type
  • This was only being used for testing. Replaced by propgator cells.

Unchanged:

• As before, render() will happily bind primitive values, OR any value that implements .sink().
  • Cell implements .sink()

Improvements:

• More browser tests
• More tests