errors: improve performance of New

See Issue golang#29382 and Issue golang#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>

src/errors/errors.go

@@ -5,12 +5,17 @@
 // Package errors implements functions to manipulate errors.
 package errors
 
+import "runtime"
+
 // New returns an error that formats as the given text.
 //
 // The returned error contains a Frame set to the caller's location and
 // implements Formatter to show this information when printed with details.
 func New(text string) error {
-	return &errorString{text, Caller(1)}
+	// Inline call to errors.Callers to improve performance.
+	var s Frame
+	runtime.Callers(2, s.frames[:])
+	return &errorString{text, s}
 }
 
 // errorString is a trivial implementation of error.