Add GDB module that defines an islprint command

Add GDB module that defines an islprint command

Add a python library that is installed next to libisl. GDB will automatically
load it and add a new gdbcommand called "islprint". This command can be used to
prettyprint isl values. It does not install an official pretty printer, as such
a printer will also pretty print stacktraces which may reference values already
freed.

Before:
(gdb) print stride
$1 = (isl_set *) 0x122e6c0

Now also possible:
(gdb) islprint stride
$1 = "{ MemRef_A[1] }"
(gdb) is stride
$1 = "{ MemRef_A[1] }"

Signed-off-by: Tobias Grosser <grosser@fim.uni-passau.de>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>