Update the ConvertToNative API documentation and strings.join

[TristonianJones]

Update the ConvertToNative API docs to clarify the call contract.

The change also introduces a refactor of the strings.join method which
does not rely on ConvertToNative, but rather uses the traits.Lister with
an expectation that all contained elements will be types.String values.

This change can be taken advantage of using ext.StringsVersion(2) if
you are using extension versioning. If you are not using extension versioning,
the change will be default enabled.

The functionality should be equivalent with the prior behavior, though there
are some limited cases where a ConvertToNative might succeed when
the new approach might fail.

Closes #688

[TristonianJones]

@jpbetz let me know if this addresses your use case, and also whether the updated functionality will work with your object types.

[jpbetz]

This looks great. Thanks @TristonianJones !