nlocs is now 16 bits wide

nlocs is now 16 bits wide

* libguile/objcodes.h (struct scm_objcode): Remove the "unused" field --
  the old "nexts" -- and expand nlocs to 16 bits.

* module/language/assembly/compile-bytecode.scm (write-bytecode): Write
  the nlocs as a uint16.

* module/language/assembly/decompile-bytecode.scm (decode-load-program):
  Decompile 16-bit nlocs. It seems this decompilation is little-endian
  :-/

* test-suite/tests/asm-to-bytecode.test: Fix up to understand nlocs as a
  little-endian value. The test does the right thing regarding
  endianness.