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saying

  make TRACE=1

prints most of the macro calls and their arguments.  It's easy to
trace new macros; see rules/trace.mk.

Also add some comments about what extra-packages is doing

The FFI GHCi import lib isn't needed as
compiler/ghci/Linker.lhs + rts/Linker.c link the
interpreted references to FFI to the compiled FFI.
We therefore filter it out so that we don't get
duplicate symbol errors.

The new flag prints out a warning if you have a local,
polymorphic binding that lacks a type signature. It's meant
to help with the transition to the new typechecker, which
discourages local let-generalisation.

At the same time I moved the missing-signature code to TcHsSyn,
where it takes place as part of zonking.  That way the 
types are reported after all typechecking is complete,
thereby fixing Trac #3696.  (It's even more important for
local bindings, which is why I made the change.)

and remove the temporary DOpt class workaround.

We need to directly depend on the stage1 libs. The stage1 compiler lib
doesn't depend on them.

rather than being repeated in the build system

This is a tricky point:
   see Note [Implicit parameter untouchables]

Improves kind error messages in paticular

See Note [Subtle shadowing in coercions]

This is what was going wrong in Trac 4160.

We weren't gathering the type variables free in the kind
of a coercion binder!