sequence of expressions -> seq of head and tail

sequence of expressions -> seq of head and tail

* libguile/expand.h:
* module/language/tree-il.scm: Rename "sequence" to "seq", and instead
  of taking a list of expressions, take a head and a tail.

* module/language/tree-il/analyze.scm:
* module/language/tree-il/compile-glil.scm:
* module/language/tree-il/fix-letrec.scm:
* module/language/tree-il/spec.scm:
* module/language/elisp/compile-tree-il.scm:
* module/ice-9/psyntax.scm:
* module/ice-9/psyntax-pp.scm:
* module/ice-9/eval.scm:
* libguile/memoize.h:
* libguile/memoize.c:
* libguile/expand.c:
* libguile/eval.c: Adapt to the new seq format.