Introduce branching sections

[jonatanklosko]

Closes #21.

Allows for having sections that branch out of a previous regular section. Such branching section still has access to all bindings defined by the parent section, but runs concurrently and generally has no impact on further sections (for instance "stale" invalidation is limited to that single section).

Intro

Demo

branching.mp4

Details

From Livbook.Session:

## Evaluation

All regular sections are evaluated in the same process
(the :main_flow evaluation container). On the other hand,
each branching section is evaluated in its own process
and thus runs concurrently.

### Implementation considerations

In practice, every evaluation container is a `Livebook.Evaluator`
process, so we have one such process for the main flow and one
for each branching section. Since a branching section inherits
the evaluation context from the parent section, the last context
needs to be copied from the main flow evaluator to the branching
section evaluator. The latter synchronously asks the former for
that context using `Livebook.Evaluator.fetch_evaluation_context/3`.
Consequently, in order to evaluate the first cell in a branching
section, the main flow needs to be free of work, otherwise we wait.
This assumptions are mirrored in by `Livebook.Session.Data` when
determining cells for evaluation.

Note: the context could be copied asynchronously if evaluator
kept the contexts in its process dictionary, however the other
evaluator could only read the whole process dictionary, thus
allocating a lot of memory unnecessarily, which would be unacceptable
for large data. By making a synchronous request to the evalutor
for a single specific evaluation context we make sure to copy
as little memory as necessary.

[jonatanklosko]

Each context now has a unique id. If we keep reevaluating the first cell in a branching section, we will transfer the context only the first time.

[jonatanklosko]

This was fun

[josevalim]

Absolutely beautiful! Just some minor comments and ship it!