Tidy up pretty-printing of SrcLoc and SrcSpan
Improve pretty-printing of type variables

In particular, print a bit of debug info in debug-style and dump-style
Otherwise distinct type variables look the same
Some refactoring around endPass and debug dumping

I forget all the details, but I spent some time trying to
understand the current setup, and tried to simplify it a bit
Simplify the generation of superclass constraints in tcInstDecl2

The simplified function is tcSuperClasses;
no need for an implication constraint here
Add the unfolding and inline-pragma for DFuns in DsBinds, not TcInstDcls

This is a straight refactoring that puts the generation of unfolding
info in one place, which is a lot tidier
White space only
Fix reduceTyFamApp_maybe

This function previously would expand *data* families even when it was asked
for a *Nominal* coercion.  This patch fixes it, and adds comments.
Simplify the API for tcInstTyVars, and make it more consistent with other similar functions
Rename setRole_maybe to downgradeRole_maybe

This change is just for naming uniformity with the existing downgradeRole
Refactor skolemising, and newClsInst

This makes newClsInst (was mkInstance) look more like newFamInst, and simplifies
the plumbing of the overlap flag, and ensures that freshening (required by
the InstEnv stuff) happens in one place.

On the way I also tided up the rather ragged family of tcInstSkolTyVars and
friends.  The result at least has more uniform naming.
Refactor the treatment of lexically-scoped type variables for instance declarations

Previously the univerally-quantified variables of the DFun were also (bizarrely)
used as the lexically-scoped variables of the instance declaration.  So, for example,
the DFun's type could not be alpha-renamed.  This was an odd restriction, which has
bitten me several times.

This patch does the Right Thing, by adding an ib_tyvars field to the
InstBindings record, which captures the lexically scoped variables.
Easy, robust, nice.  (I think this record probably didn't exist originally,
hence the hack.)
Get the Untouchables level right in simplifyInfer

Previously we could get constraints in which the untouchables-level did not
strictly increase, which is one of the main invariants!

This patch also simplifies and modularises the tricky case of generalising
an inferred let-binding
Normalise the type of an inferred let-binding

With the new constraint solver, we don't guarantee to fully-normalise
all constraints (if doing so is not necessary to solve them). So we
may end up with an inferred type like
      f :: [F Int] -> Bool
which could be simplifed to
      f :: [Char] -> Bool
if there is a suitable family instance declaration.  This patch
does this normalisation, in TcBinds.mkExport
Typechecker debug tracing only
When reporting the context of given constraints, stop when you find one
that binds a variable mentioned in the wanted

There is really no point in reporting ones further out; they can't be useful
Only report "could not deduce s~t from ..." for givens that include equalities

This just simplifies the error message in cases where there are no useful
equalities in the context
Don't filter out allegedly-irrelevant bindings with -dppr-debug
Minor refactoring (no change in functionality)
Define ctEvLoc and ctEvCoercion, and use them
Test Trac #9211