* error.h, error.c: Added `scm_wrong_type_arg_msg' to support

* error.h, error.c: Added `scm_wrong_type_arg_msg' to support
displaying the expected type.  Use SCM_LISTn in a couple places
instead of scm_cons-ing by hand.

* __scm.h: Added SCM_ASSERT_TYPE macro.

* validate.h, scm_validate.h: Added the former, as a renamed
version of the latter with SCM_ASSERT_TYPE used in
SCM_MAKE_VALIDATE (instead of just SCM_ASSERT)

* Makefile.am: Rename scm_validate.h to validate.h.

* *.c, *.h: Include validate.h, not scm_validate.h (old name's
prefix was superfluous).