arrows is the only library in libraries/extra-packages not built when present

This should result in successful program termination. Not doing this
breaks checking the exit code of  runghc Setup.lhs build  for example.

Avoid:
    let x ...
i.e. always put the = in:
    let x = ...

Used to just give a "Non-exhaustive patterns" failure

Argubly rewrite rules should not fire with -O0, and it turns
out that when compiling GHC.Base with -O0 we get a crash if
the rewrite rules do fire (see Note [Scoping for Builtin rules]
in PrelRules).

So unless someone yells, rewrite rules are off with -O0.

The new (now dynamic) flag is 
    -frewrite rules (with -fno-rewrite-rules to disable)

The old (static) flag -frules-off is gone.

This fixes Trac #1251; test case is gadt/CasePrune

GHC was being over-eager about pruning dead alternatives from case
expressions, and that led to a crash because the case expression 
ended up with no alternatives at all!

See the long comments Note [Pruning dead case alternatives] in Unify.

isDataTyCon advertises that it's true of "data types that are
definitely represented by heap-allocated constructors.  These are
srcutinised by Core-level @case@ expressions, and they get info tables
allocated for them."

Type-family TyCons never have this property, not even data type families.
It's the *instance* TyCons that do.

I hope that this change does not break anything that somehow relied
on the old (wrong) semantics.

A SpecInfo (inside IdInfo) keeps track of the free variables of a RULE
so that the occurrency analyser knows about its dependencies.

Previously it was only tracking the *rhs* free vars, but it should
really include the *lhs* ones too.  See Note [Rule dependency info]
in IdInfo.

This fixes a WARNING when compiling some libraries.

cvts[sd]2siq? ->
cvtts[sd]2siq?

This means we can avoid some StablePtrs, and also catch cases where
the AP_STACK has been evaluated (this can happen with :history, see
the hist001 test).

Now you can list source code in various ways:
  :list <line>
  :list <module> <line>
  :list <function>

We don't touch $@ if configure failed as we would prefer to try
configuring it next time round, rather than assuming it'll still fail.
This is particularly important for corelibs, where failure means the
build dies!

The debugger can now log each step of the evaluation without actually
stopping, keeping a history of the recent steps (currently 50).  When
a (real) breakpoint is hit, you can examine previous steps in the
history (and their free variables) using the :history, :back and
:forward commands.

Use the right number of arguments in a panic case.

-ticky is the command-line flag for ticky-ticky profiling now, but
internally, the -fticky-ticky flag was used to represent whether ticky
was on. This led to link errors if the user supplied -fticky-ticky but
not -ticky (ticky code would be generated but the wrong RTS library
would be passed to the linker). I removed references to -fticky-ticky
-- now, only the way flags dictate whether ticky is on, and
-fticky-ticky is no longer accepted as a command-line option.