Add support for stringref extension

[sffc]

The stringref extension for CBOR allows for strings to be deduplicated (referenced from a common source). This is useful if you have a data structure with a lot of duplicated strings.

http://cbor.schmorp.de/stringref

This was previously attempted at #60, but that PR was abandoned.

My use case: I am dealing with CLDR data, which has a whole lot of duplicated information. gzip doesn't fit my use case, because I want to point at the strings in shared memory, rather than unpacking the strings from gzip and requiring that they have an owner.