log/slog: use unsafe.StringData, SliceData

log/slog: use unsafe.StringData, SliceData

Use the new functions in 1.20 (unsafe.StringData, etc.) instead
of StringHeader and StructHeader from the reflect package.

Updates golang/go#56345.

Change-Id: I84d0db7b203aeffe45ce8b06beb7b4ee17e19949
Reviewed-on: https://go-review.googlesource.com/c/go/+/478055
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>