* Introduces the `SymbolRef` type, a borrowed view of a text
  value for field lookups.
* Introduces the `AsSymbolRef` trait to allow values to be
  temporarily viewed as a `SymbolRef`.
* Modifies the `IonStruct` trait to use `AsSymbolRef` for lookups
  instead of `AsRef<str>`. This change enables lookups for unknown
  text.
* Modifies the `StructRef` type to use `AsSymbolRef` for its lookup
  methods.
* Modifies the `Struct` type to use `AsSymbolRef` for its lookup
  methods.
* Modifies the `Struct` type to store its values in a `SmallVec`,
  improving the performance of the common case. The first field
  value for any given field name will be stored directly in the
  map. If the field name is repeated, the additional values will
  be stored elsewhere on the heap. Prior to this change, *all*
  field values were stored on the heap in a `Vec`.
* Adds `len()` and `is_empty()` methods to the `Struct` type.
  We will likely wish to move these into the `IonStruct` trait
  in the future.
* Tweaks the ion-tests test runner to print the Ion text of the
  elements that caused test failures instead of using their
  auto-generated `Debug` format.