log/slog: optimize slog Level.String() to avoid fmt.Sprintf

log/slog: optimize slog Level.String() to avoid fmt.Sprintf

No more overhead of fmt.Sprintf, especially beneficial for
high-frequency logging.

goos: linux
goarch: amd64
pkg: log/slog
cpu: Intel(R) Xeon(R) Gold 6133 CPU @ 2.50GHz
              │ /tmp/old.txt │            /tmp/new.txt             │
              │    sec/op    │   sec/op     vs base                │
LevelString-4   1006.5n ± 2%   334.2n ± 1%  -66.80% (p=0.000 n=10)

              │ /tmp/old.txt │            /tmp/new.txt            │
              │     B/op     │    B/op     vs base                │
LevelString-4     56.00 ± 0%   48.00 ± 0%  -14.29% (p=0.000 n=10)

              │ /tmp/old.txt │          /tmp/new.txt          │
              │  allocs/op   │ allocs/op   vs base            │
LevelString-4     7.000 ± 0%   7.000 ± 0%  ~ (p=1.000 n=10) ¹

Change-Id: I6585d4aaa4da55d72ac70bc66dff45500eccd056
Reviewed-on: https://go-review.googlesource.com/c/go/+/704975
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Auto-Submit: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>