internal/coverage/slicereader: use unsafe.String

internal/coverage/slicereader: use unsafe.String

Change-Id: Ifd88315626e0ce5b9b8d3c66aa9dd9806f8407dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/448555
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Than McIntosh <thanm@google.com>
Auto-Submit: Than McIntosh <thanm@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>