Apply some improvements on Aaron Hawley's long-overdue patch to the
Info mode. TAB now cycles through references (menu items and cross
references); M-TAB cycles backward; RET by default runs
`info-current-menu-item', without having to customize the
`info-selection-key' variable; and all of the relevant regular
expressions now are defined at the top level, not hard-coded as
literals in the procedures that refer to them, and use the REXP
abstraction for clarity, since I couldn't follow the baroque string
notation without painstakingly rewriting it character by character.