Add walk built-in function

While working on Terraform policies we encountered JSON documents that
can be nested arbitrarily, e.g.:

foo:
  bar:
    baz:
     v: 1
    qux:
     v: 2
  corge:
    v: 3

In the Terraform case, we want to process objects at foo.bar.baz,
foo.bar.qux, and foo.corge, however, we don't know ahead of time what
these paths will be.

To handle this, we add a new built-in function 'walk' that traverses all
documents nested under the input.