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Coalescences must be applied to the unsimplified graph in the same order 
they were found by the iterative coalescing algorithm - otherwise
the vreg rewrite mapping will be wrong and badness will ensue.

When -dasm-lint is turned on the register conflict graph is checked for 
internal consistency after each build/color pass. Make sure that all 
edges point to valid nodes, that nodes are colored differently to their
neighbours, and if the graph is supposed to be colored, that all nodes
actually have a color.

To help with debugging / nicer -ddump-asm-regalloc-stages

On further testing it turns out that using Chaitin's spill cost formula
of counting the number of uses of a var and then dividing by the degree
of the node isn't actually a win. Perhaps because this isn't counting 
the number of actual spills inserted in the output code.

This would be worth revisiting if other work manages to increase the 
register pressure in the native code.

Track what slots each basic block reloads from. When cleaning spill 
instructions we can use this information to decide whether the slot 
spilled to will ever be read from on this path.

This lets us gain coverage on programs that use fork, like xmonad.

* To be merged to STABLE *

* To be merged to STABLE. *

This change allows you to write
	f :: (Eq a) => (Ord b) => a -> b -> b
Previously you could only have a forall and context after '->'
but not after '=>' which is strange and inconsistent.

Making the parser a bit more generous didn't change the number
of shift/reduce conflicts.

tc236 tests.

- checkTauTvUpdate now distinguishes between whether
  (1) a type variables occurs only in type family parameters
      (in which case unification is to be deferred)
  (2) other variable occurences
      (which case we fail with a cannot create infinite type message, as before)

Fill all of the sigevent structure with zeroes before individual fields are
set. Although not strictly necessary, this keeps tools like valgrind from
complaining about passing uninitialized data, which is a good thing.

MERGE TO STABLE

installPackage is a horror, using --force is probably not the perfect way
to make DESTDIR functionality work again, but given the current state of
affairs, I can't find a quick and clean solution.

MERGE TO STABLE

Having all package directory directly in the toplevel installation directory
would be a bad thing because of potential name clashes, and it wasn't done this
way earlier, so reinstantiate the old layout.

MERGE TO STABLE

- Cleaned up and simplified rules
- Added many missing cases
- The rules OccursCheck and swap have been eliminated and integrate with
  the other rules; ie, Subst and Unify perform the occurs check themselves
  and they can deal with left-to-right and right-to-left oriented rewrites.
  This makes the code simpler and more efficient.
- Also added comments.

Watch out: I've added MkZipCfg.sequence, which clashes with the
Prelude (like some other names in MkZipCfg.  Maybe you can think
of a better name for it.