Flatten Pest to Ion `sequence`/`choice` Nodes

[almann]

The sequence/choice nodes in the Pest grammar AST are binary. We should allow them to flatten such that:

(choice
  (choice
    (identifier "a")
    (identifier "b")
  )
  (identifier "c")
)

Turn into:

(choice
  (identifier "a")
  (identifier "b")
  (identifier "c")
)

This makes downstream consumption easier. Note that sequence has higher precedence than choice.