errors: improve performance of New

See Issue #29382 and Issue #30468.

Improvements in this CL:

name                     old time/op  new time/op  delta
New-8                     352ns ± 2%   225ns ± 5%  -36.04%  (p=0.008 n=5+5)

Improvements together with moving to 1 uintptr:

name                     old time/op  new time/op  delta
New-8                     475ns ± 3%   225ns ± 5%  -52.59%  (p=0.008 n=5+5)

Change-Id: I9d69a14e5e10a6498767defb7d5f26ceedcf9ba5
Reviewed-on: https://go-review.googlesource.com/c/go/+/167401


Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>