The Grow method is like the proposed slices.Grow function
in that it ensures that the slice has enough capacity to append
n elements without allocating.
The implementation of Grow is a thin wrapper over runtime.growslice.
This also changes Append and AppendSlice to use growslice under the hood.
Fixes #48000
Change-Id: I992a58584a2ff1448c1c2bc0877fe76073609111
Reviewed-on: https://go-review.googlesource.com/c/go/+/389635
Run-TryBot: Joseph Tsai <joetsai@digital-static.net> Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>