* conv-integer.i.c, conv-uinteger.i.c: New files, used to generate

* conv-integer.i.c, conv-uinteger.i.c: New files, used to generate
the functions below.

* numbers.c, numbers.h (scm_to_int8, scm_to_uint8, scm_to_int16,
scm_to_uint16, scm_to_int32, scm_to_uint32, scm_to_int64,
scm_to_uint64, scm_from_int8, scm_from_uint8, scm_from_int16,
scm_from_uint16, scm_from_int32, scm_from_uint32, scm_from_int64,
scm_from_uint64): Turned from macros into proper functions.
(scm_to_signed_integer, scm_to_unsigned_integer,
scm_from_signed_integer, scm_from_unsigned_integer): Generate via
conv-integer.i.c and conv-uinteger.i.c, as well.