Hi guix,

The two atached patches optimise bytevector->nix-base32-string and
bytevector->base16-string, making them about 20% and two times
faster respectively, by reducing allocations.  They are called
from 'output-path', 'fixed-output-path' and 'store-path'
in (guix store).

Unfortunately, this does not decrease timings to a noticable degree,
but it does decrease the allocated memory by 2.3% (*), and it does not
seem to increase timings.  (See perf-numbers.txt.)

(*) GUIX_PROFILING=gc guix build -d pigx --no-grafts

Greetings,
Maxime.