Major change in compilation of instance declarations (fix Trac #955, #2328)

Major change in compilation of instance declarations (fix Trac #955, #2328)

This patch makes an important change to the way that dictionary
functions are handled.  Before, they were unconditionally marked
INLIINE, but all the code written by the user in the instance
was inside that unconditionally-inlined function.  Result: massive
code bloat in programs that use complicated instances.

This patch make instances behave rather as if all the methods
were written in separate definitions.  That dramatically reduces
bloat.  The new plan is described in TcInstDcls
	Note [How instance declarations are translated]

Everything validates.  The major code-bloat bug is squashed: in particular
DoCon is fine now (Trac #2328) and I believe that #955 is also better.

Nofib results:

Binary sizes
        -1 s.d.      +2.5%
        +1 s.d.      +3.1%
        Average      +2.8%

Allocations
        -1 s.d.      -6.4%
        +1 s.d.      +2.5%
        Average      -2.0%

Note that 2% improvement.  Some programs improve by 20% (rewrite)!
Two get slightly worse: pic (2.1%), and gameteb (3.2%), but all others
improve or stay the same.

I am not absolutely 100% certain that all the corners are correct; for
example, when default methods are marked INLINE, are they inlined?  But
overall it's better.

It's nice that the patch also removes a lot of code.  I deleted some
out of date comments, but there's something like 100 fewer lines of
code in the new version!  (In the line counts below, there are a lot
of new comments.)