* lisp/emacs-lisp/bindat.el: Minor refactoring

* lisp/emacs-lisp/bindat.el: Minor refactoring

(bindat--unpack-str, bindat--unpack-strz, bindat--unpack-bits):
New functions, extracted from `bindat--unpack-item`.
(bindat--unpack-item): Use them.
(bindat--align): New function.
(bindat--unpack-group, bindat--length-group, bindat--pack-group): Use it.
(bindat-get-field): Allow integers to index both lists (as returned by
`repeat`) and vectors (as returned by `vec`).
(bindat--pack-str, bindat--pack-bits): New functions, extracted from
`bindat--pack-item`.
(bindat--pack-item): Use them.

* test/lisp/emacs-lisp/bindat-tests.el (struct-bindat): Place the fields
in the order in which they appear in the structs.