* eval.c (SCM_CEVAL): Cleaned up the handling of 'if', 'let',

* eval.c (SCM_CEVAL): Cleaned up the handling of 'if', 'let',
'letrec' and 'set*': Removed some uses of t.arg1, t.lloc and proc
as temporary variables.  Removed side-effecting operations from
conditions and macro calls.  Introduced temporary variables with
hopefully descriptive names for clarification.  Replaced SCM_N?IMP
by a more explicit predicate in some places.  Removed code that
was conditionally compiled if SICP was defined - which it never
is.