Transform CONSP to LISTP for known not-nil argument.
Move RELEASE-FD-STREAM-BUFFER after the defstruct for FD-STREAM.

This inlines the accessors. Similarly STREAM-OUTPUT-STREAM can
avoid calling specifier-type in its typecase.
Use %OTHER-POINTER-WIDETAG instead of WIDETAG-OF where applicable.

Also strengthen a declaration in DUMP-SPECIALIZED-VECTOR.
Delete obsolete {GET,SET}-VECTOR-SUBTYPE and -WORD-LENGTH vops.