info: add getters for make, model and serial

The functionality of these new high-level functions as implemented here
follows what Weston 11.0's ad hoc EDID parser extracted. There are
differences to Weston though:
- get_model() falls back to the numeric model field
- unusual characters are escaped as hex rather than replaced with dash

The documentation of the new functions allows to change the behavior of
get_model and get_make to e.g. convert PNP ID to a company name. The
notes about UTF-8 allow us to make use of UTF-8 fields if such appear
useful in the future.

Weston's ad hoc parser shall be replaced with this. After another API
addition, the high-level API semantics will match what Weston expects.

The memory_stream stuff is for convenience and will see more complex
use in the future.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>