factor copy-tree and cons-source out of eval.[ch]

factor copy-tree and cons-source out of eval.[ch]

* libguile.h:
* libguile/Makefile.am
* libguile/init.c (scm_i_init_guile): Add trees.[ch] to the build.

* libguile/eval.c:
* libguile/eval.h: Remove scm_copy_tree and scm_cons_source...

* libguile/trees.h:
* libguile/trees.c:
* libguile/srcprop.h:
* libguile/srcprop.c: ... factoring them out here and here,
  respectively.

* test-suite/tests/eval.test ("memoization"): Change expected exception
  for circular data structures, given new copy-tree location.