[uuid] Add uuid_aton() to parse a UUID from a string

[uuid] Add uuid_aton() to parse a UUID from a string

Add uuid_aton() to parse a UUID value from a string (analogous to
inet_aton(), inet6_aton(), sock_aton(), etc), treating it as a
32-digit hex string with optional hyphen separators.  The placement of
the separators is not checked: each byte within the hex string may be
separated by a hyphen, or not separated at all.

Add dedicated self-tests for UUID parsing and formatting (already
partially covered by the ":uuid" and ":guid" settings self-tests).

Signed-off-by: Michael Brown <mcb30@ipxe.org>