tools: reduce copies b/w ocaml Strings and Bytes

tools: reduce copies b/w ocaml Strings and Bytes

When xenstore was ported to the new safe-string interface, it mostly
happened by making copyies of string into bytes and back.  The ideal
fix would be to rewrite all of the relevant interfaces to be uniformly
using bytes, but in the meanwhile we can improve the code by using unsafe
conversion functions (see
 https://caml.inria.fr/pub/docs/manual-ocaml/libref/Bytes.html#3_Unsafeconversionsforadvancedusers).

In most cases we own the bytes that we are converting to string, or we
immediately make copies that we then mutate, or we use them immutably
as payloads for writes. In all these cases it is safe to use the unsafe
functions and prevent a copy.

This patch updates the code to use the unsafe conversions where possible.

Signed-off-by: Marcello Seri <marcello.seri@citrix.com>
Reviewed-by: Christian Lindig <christian.lindig@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>