A first attempt to capture the type changes.

Instead of trying to make open arrays look like arrays (which they aren't,
they just look like them), I decided to call them "slice types". As a result
they have value semantics, and one can really talk about what they really are.

Note:
- There are lots of missing details
- There are probably lots of mistakes

Looking for some initial feedback.

R=r
DELTA=444  (180 added, 189 deleted, 75 changed)
OCL=21769
CL=22020