add metalen field to bytecode serialization

add metalen field to bytecode serialization

* libguile/objcodes.h (struct scm_objcode): Add a new field, metalen, in
  preparation for embedding metadata within a program.
  (SCM_OBJCODE_META_LEN, SCM_OBJCODE_TOTAL_LEN): New defines.

* libguile/vm.c (really_make_boot_program):
* module/language/assembly.scm (*program-header-len*, byte-length):
* module/language/assembly/compile-bytecode.scm (write-bytecode):
* module/language/assembly/decompile-bytecode.scm (decode-load-program):
* module/language/assembly/disassemble.scm (disassemble-load-program):
* module/language/glil/compile-assembly.scm (glil->assembly):
* test-suite/tests/asm-to-bytecode.test ("compiler"): Update for metalen
  addition.