Removing negative prims

Hi All

prompted by the Diskussion with James Cook on @darcs-users I noted that
all Contexted patches in Darcs are positive throughout i.e. including
the context. This led me to investigate whether we could eliminate
inversion for named prim patches in a way similar to what we did for
Named patches at the repo level, thereby turning what previously was
(merely) an invariant into something that is baked into the type.

So I have made an experimental change that removes the ability to invert
PrimPatchIds, and consequently removes class SignedId and the Invert
instance for PrimWithName. For RepoPatchV3 this means we must store the
patches that a Conflictor reverts in the backward direction and invert
them only after stripping the names (i.e. in the implementation of
'effect'). This change requires a new class CleanUnmerge that does the
opposite of CleanMerge. It also requires new functions in the Contexted API.

I can compile and successfully run all tests (though the tests for named
prims of course have to be stripped down because we no longer have
inversion for them).

The question is: is this something worthwhile?

It does simplify the invariants: we no longer have to test that
Contexted patches and Conflictors are positive, because negative named
prims no longer exist. It does not simplify the code in any significant
way, but it arguably clarifies some details in the RepoPatchV3
implementation; for instance it now becomes obvious at which points we
add prims to a context and where we remove them.

I don't expect, nor have I seen, any noticeable impact on performance.
It would change the on-disk representation of darcs-3 patches. In fact,
this makes RepoPatchV3 a bit more like RepoPatchV2 which also stores the
effect of a Conflictor in non-inverted form.

I can send a patch to make this a bit more concrete if there is interest.

Cheers
Ben