ELF refactor and consequent linker simplifications

* module/system/vm/elf.scm: Add commentary.
  (make-elf): Add a constructor similar to make-elf-segment and
  make-elf-section.
  (write-elf32-header, write-elf64-header, write-elf-header): Take an
  <elf> instead of all the fields separately.
  (<elf-segment>, <elf-section>): Add "index" property.  Adapt
  constructors accordingly.

* module/language/objcode/elf.scm (bytecode->elf): Arrange to set the
  section indexes when creating ELF sections.

* module/system/vm/linker.scm (fold-values): New helper.
  (alloc-segment, relocate-section-header): Arrange to set segment and
  section indexes.
  (find-shstrndx): New helper, replaces compute-sections-by-name.  Now
  that sections know their indexes, this is easier.
  (allocate-elf, write-elf): New helpers, factored out of link-elf.
  Easier now that sections have indexes.
  (link-elf): Simplify.  Check that the incoming objects have sensible
  numbers.

* test-suite/tests/linker.test: Update to set #:index on the linker
  objects.

test-suite/tests/linker.test

@@ -31,9 +31,10 @@
         (lambda (table idx)
           (set! string-table table)
           idx)))
-    (define (make-object name bv relocs . kwargs)
+    (define (make-object index name bv relocs . kwargs)
       (let ((name-idx (intern-string! (symbol->string name))))
         (make-linker-object (apply make-elf-section
+                                   #:index index
                                    #:name name-idx
                                    #:size (bytevector-length bv)
                                    kwargs)
@@ -41,11 +42,11 @@
                             (list (make-linker-symbol name 0)))))
     (define (make-string-table)
       (intern-string! ".shstrtab")
-      (make-object '.shstrtab (link-string-table string-table) '()
+      (make-object 2 '.shstrtab (link-string-table string-table) '()
                    #:type SHT_STRTAB #:flags 0))
     (let* ((word-size (target-word-size))
            (endianness (target-endianness))
-           (sec (make-object name bytes '()))
+           (sec (make-object 1 name bytes '()))
            ;; This needs to be linked last, because linking other
            ;; sections adds entries to the string table.
            (shstrtab (make-string-table)))