add scm_c_make_struct[v]

add scm_c_make_struct[v]

* libguile/struct.h (scm_c_make_struct, scm_c_make_structv): New
  functions with which you can make a struct without consing a rest
  list.

* libguile/struct.c (scm_struct_init): Refactor to take an array of init
  values, not a list.
  (scm_make_struct, scm_make_vtable_vtable): Pull the rest arg out into
  a list and pass it down to the new array-taking functions.

* libguile/memoize.c: Remove a neeless #include <alloca>.