[util] Update constexpr hash to be FNV-1a

The constexpr_hash.h header has fallen out of use. As constexpr hashing
will be used for IDs with the service locator protocol, update these
hashes to be 32 and 64 bit FNV-1a, and replace the _h user-defined
literal with _id (a 64-bit hash), and _id8 (a 32-bit hash folded down to
8 bits). These are now in the util/hash.h header along with the runtime
hash functions.

src/user/drv.uart/main.cpp

@@ -9,7 +9,6 @@
 #include <j6/syscalls.h>
 #include <j6/sysconf.h>
 #include <j6/types.h>
-#include <util/constexpr_hash.h>
 
 #include "io.h"
 #include "serial.h"