http2: use an array instead of a map in typeFrameParser

FrameType is a dense integer range, so we can store the frameParsers
in an array instead of a map. This should be a very small performance
win on all Go http2 servers. For high QPS gRPC services, this function
is visible in the Go profiler. For example, it shows up as 0.16% of
all CPU time on one production service at Datadog.

Change FrameType.String() to use the same pattern.

Add a test for testFrameType with unknown FrameTypes.

Fixes golang/go#73613

Change-Id: I5f5b523e011a99d6b428cbdbfd97415e488169d1
Reviewed-on: https://go-review.googlesource.com/c/net/+/670415
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>