Top-level containers are no longer followed by a comma

[zslayton]

This PR depends on #30. Once 30 has been merged, this one will be rebased against master.

---

PR #28 attempted to fix #17, but overcorrected.

Previously, nested containers would not be followed by a delimiting ,:

[
  {}
  {}
  {}
]

Following #28, this was fixed:

[
  {},
  {},
  {},
]

but it also added commas after top-level containers:

$ion_symbol_table::{
  // ....
},
{
  foo: true
},
{
  bar: 5
},

which is incorrect.

This PR fixes that behavior.
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