* guix/build/graft.scm (replace-store-references): Reimplement for
faster grafting. Use binary I/O instead of textual I/O. Replace
'mapping' argument (an alist) with 'replacement-table' (a vhash).
(rewrite-directory): Adapt to mapping argument change in
'replace-store-references'. Remove 'with-fluids' that previously set
'%default-port-encoding' to #f, since we now use binary I/O.
(define-inline, hash-length): New macros.
(nix-base32-char?): New variable.
5a1add373a