encoding/asn1: remove allocation from init

encoding/asn1: remove allocation from init

asn1 allocates due to reflect.TypeOf(new(big.Int)) in init time.
We could replace it with (*big.Int)(nil).

Before:
init encoding/asn1 @1.0 ms, 0.009 ms clock, 224 bytes, 7 allocs

After:
init encoding/asn1 @0.70 ms, 0.002 ms clock, 192 bytes, 6 allocs

Fixes #55973

Change-Id: I7c3cc0f48631af73cf34ad3c731c380f46c72359
Reviewed-on: https://go-review.googlesource.com/c/go/+/435257
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: hopehook <hopehook@golangcn.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>