Change type of zed.nativeBase for Go 1.21

Unlike previous releases, Go 1.21 assigns zed.nativeBase (a struct{}
occupying zero bytes) the same address as runtime.zerobase, causing
panics because zed.decodeNative now mistakes a zed.Value with a bytes
field containing a zero-capacity slice for a native value.  Fix by
changing the type of nativeBase to uintptr, which does not occupy zero
bytes.

Closes #4764.

value.go

@@ -157,7 +157,7 @@ func (v *Value) Bytes() zcode.Bytes {
 // nativeBase is the base address for all native Values, which are encoded as a
 // zcode.Bytes with this base address, a length of zero, and capacity set to the
 // bits of the value's native representation.
-var nativeBase struct{}
+var nativeBase uintptr
 
 func encodeNative(x uint64) zcode.Bytes {
 	var b zcode.Bytes