* pairs.h, eval.c, gc.c, init.c, ioext.c, ports.c, ports.h,

* pairs.h, eval.c, gc.c, init.c, ioext.c, ports.c, ports.h,
srcprop.h, tags.h, throw.c, unif.c: Added new selectors
SCM_SETAND_CAR, SCM_SETAND_CDR, SCM_SETOR_CAR and SCM_SETOR_CDR.
Motivation: Safer use.  Some other macros are defined in terms of
these operations.  If these are defined using the SCM_SETCXR
(<e1>, SCM_CXR (<e1>) <op> <e2>) pattern a complex <e1> will lead
to inefficiency and an <e1> with side-effects could potentially
break.  Also, these particular operations are heavily utilized in
the garbage collector.  In unoptimized code there will be a
measurable speedup.

* alist.c, arbiters.c, continuations.c, debug.c, debug.h, eval.c,
eval.h, feature.c, filesys.c, fports.c, gc.c, gsubr.c, init.c,
ioext.c, kw.c, list.c, load.c, mallocs.c, numbers.c, numbers.h,
pairs.c, pairs.h, ports.c, ports.h, posix.c, procprop.c, procs.c,
procs.h, ramap.c, read.c, root.c, srcprop.c, srcprop.h,
strports.c, symbols.c, tags.h, throw.c, unif.c, variable.c,
vports.c: Cleaned up use of pairs: Don't make any special
assumptions about the internal structure of selectors and
mutators: SCM_CXR (<e1>) = <e2> --> SCM_SETCXR (<e1>, <e2>),
SCM_CXR (<e1>) &= <e2> --> SCM_SETAND_CXR (<e1>, <e2>) etc.
(Among other things, this change makes it easier to build Guile
with certain compilers which have problems with casted lvalues.)