gdb-mi: Don't use bindat for field access

gdb-mi: Don't use bindat for field access

Replace uses of bindat-get-field with a simpler accessor,
since nothing here uses the bindat machinery in any way.

* lisp/progmodes/gdb-mi.el (gdb-mi--field): New.
(gdb-get-many-fields): Remove.
(gdb-var-create-handler, gdb-var-list-children-handler)
(gdb-var-update-handler, gdb-current-buffer-frame)
(gdb-update-gud-running, gdb-thread-exited, gdb-thread-selected)
(gdb-running, gdb-stopped, gdb-breakpoints-list-handler-custom)
(gdb-place-breakpoints, gdb-thread-list-handler-custom)
(def-gdb-thread-buffer-simple-command, gdb-select-thread)
(def-gdb-thread-buffer-gud-command, gdb-read-memory-custom)
(gdb-invalidate-disassembly, gdb-disassembly-handler-custom)
(gdb-disassembly-place-breakpoints, gdb-toggle-breakpoint)
(gdb-delete-breakpoint, gdb-goto-breakpoint, gdb-frame-location)
(gdb-stack-list-frames-custom, gdb-select-frame)
(gdb-edit-locals-value, gdb-locals-handler-custom)
(gdb-registers-handler-custom, gdb-changed-registers-handler)
(gdb-register-names-handler, gdb-frame-handler):
Use gdb-mi--field.