linker string tables are stateful objects

linker string tables are stateful objects

* module/system/vm/linker.scm (make-string-table): Rework to be a
  stateful object instead of a function object.  Works better in this
  case.  Adapt users.
  (string-table-intern!): Rename from string-table-intern, and just
  return the index of the string.
  (link-string-table!): Rename from link-string-table, and set a flag to
  prevent interning strings after linking, as that's not going to work
  well.

* module/language/objcode/elf.scm (bytecode->elf): Adapt.