Add an EXI yang-data codec

We already have yang-data-codec-xml, but for persisting data in a packed format EXI offers significant benefits. While both EXI implementations offer a relatively easy way of plugging into javax.xml APIs, it would be useful to package such a codec out of the box.

The implementation should be hosted in yang-data-codec-exificient, which will use yang-data-codec-xml and https://github.com/EXIficient/exificient-core to provide such functionality.

Initial implementation should use just the base XSD schema for encoding and allow specification of how the stream should be packed (default to byte-aligned, provide options to use bit-packed and precompress modes).

An example of integration is available in https://git.opendaylight.org/gerrit/#/c/35131/ and surrounding code.