-----------------
	Make seq built in
	-----------------

	DO NOT merge with stable

Until this commit 'seq' used a cunning hack so that it
seems to be *non-strict* in its second argument:

  seq x y = case seq# x of { 0 -> y; other -> error "urk" }

The reason for this is to make sure that x is evaluated before y,
which is what you want in a parallel setting.

But in a *sequential* settting, this simply destroys strictness
information about y.  Now that people are starting to use seq more,
this is becoming painful.  People sometimes use seq to make their
function strict, and are surprised when it becomes non-strict in other
arguments!

So this commit changes seq so that it does what you would naively
expect:

	seq x y = case x of { any -> y }

This is done by making seq built-in, defined along with
	unsafeCoerce
	getTag

in MkId.lhs.  (I considered giving their unfoldings in
PrelGHC.hi-boot.pp, but then there is the matter of making sure they
are unfolded, since these fns don't have top-level curried defns,
so I held off and did seq the same way as the other two.)

I renamed PrelConc.seq as PrelConc.pseq; maybe its name will change
to `then` or `before` or something else.  That's up to the GPH
folk.