slices: optimize Index and Compact for large types

slices: optimize Index and Compact for large types

Using `for i, v := range` loops causes extra copies.
Try to get rid of as much copies as possible.

  goos: windows
  goarch: amd64
  pkg: slices
  cpu: AMD Ryzen Threadripper 2950X 16-Core Processor
                       │   old.txt~    │               new.txt~               │
                       │    sec/op     │    sec/op     vs base                │
  EqualFunc_Large-32       1.077m ± 1%   1.072m ±  1%        ~ (p=0.631 n=10)
  Index_Large-32         346.329µ ± 1%   6.510µ ± 24%  -98.12% (p=0.000 n=10)
  IndexFunc_Large-32       502.9µ ± 0%   381.2µ ±  1%  -24.21% (p=0.000 n=10)
  Compact_Large-32         409.5µ ± 1%   145.2µ ±  9%  -64.54% (p=0.000 n=10)
  CompactFunc_Large-32     693.5µ ± 1%   663.1µ ±  3%   -4.39% (p=0.000 n=10)
  geomean                  556.3µ        191.3µ        -65.61%

Change-Id: I36a2f2172cf30d97a5aa6f8d7cf6981d67daec62
Reviewed-on: https://go-review.googlesource.com/c/go/+/486235
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>