break up access relations into may_read/may_write/must_write

break up access relations into may_read/may_write/must_write

Internally, we currently only make a distinction between may accesses and
must accesses.  At the level of an individual access expression, we even only
have a single (possibly argument dependent) access relation.
If we want to keep track of the accesses performed by a function called
from a function call, we will need to make a further distinction between
reads and writes.  Furthermore, if member accesses are involved, then
a function may access elements from different spaces through the same
argument.  The access relations therefore need to be changed into union maps.

We already changed the ouside interface with respect to these changes.
Now we make the internal represenation follow suit.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>