Avoid quadratic checking of identity-constraints

key/unique/keyref schema attributes currently use qudratic loops
to check their various constraints (that keys are unique and that
keyrefs refer to existing keys).  That becomes extremely slow if
there are many elements with keys.  This happens in the wild with
e.g. the OVAL XML descriptions of security patches.  You need the
openscap schemata, and then an example xml file:

% zypper in openscap-utils
% wget ftp://ftp.suse.com/pub/projects/security/oval/opensuse.leap.15.1.xml
% time xmllint --schema /usr/share/openscap/schemas/oval/5.5/oval-definitions-schema.xsd opensuse.leap.15.1.xml > /dev/null
opensuse.leap.15.1.xml validates

real    16m59,857s
user    16m55,787s
sys     0m1,060s

This patch makes libxml use a hash table to avoid the quadratic
behaviour.  The existing hash table only accepts strings as keys, so
we're mostly reusing the canonical representation of key values to derive
such strings (with the caveat given in a comment).  The alternative
would be to rework the hash table code to accept either numbers or free
functions as hash workers, but the code is fast enough as is.

With the patch we have this then:

% time LD_LIBRARY_PATH=./libxml2/.libs/ ./libxml2/.libs/xmllint --schema /usr/share/openscap/schemas/oval/5.5/oval-definitions-schema.xsd opensuse.leap.15.1.xml > /dev/null
opensuse.leap.15.1.xml validates

real    0m3,531s
user    0m3,427s
sys     0m0,103s

So, a ~300x speedup.  This patch survives 'make check' and 'make tests'.
4 jobs for faster-idc in 3 minutes and 50 seconds
latest
Status Job ID Name Coverage
  Test
passed #999173
clang:asan

00:03:49

passed #999174
clang:msan

00:03:35

passed #999171
gcc

00:01:56

passed #999172
gcc:python3

00:02:17