Add FFI support for `short' and `unsigned short'.

Add FFI support for `short' and `unsigned short'.

* libguile/foreign.c (sym_short, sym_unsigned_short): New variables.
  (scm_init_foreign): Define Scheme variables SYM_SHORT and
  SYM_UNSIGNED_SHORT.

* module/system/foreign.scm (short, unsigned-short): New exports.
  (integer-ref, integer-set): Support (= (sizeof TYPE) 2).
  (%read-short, %write-short!, %read-unsigned-short,
  %write-unsigned-short!): New variables.
  (*writers*): Add support for `short' and `unsigned-short'.
  (*readers*): Likewise.

* test-suite/tests/foreign.test ("structs")["int8, pointer, short,
  double"]: New test.