net/url: improve URL.String performance

net/url: improve URL.String performance

URL.String performs memory allocation many times, but it can improve
performance by allocating memory that it clearly knows it needs.
This CL achieves 24.6% speedup, 18.3% memory reduction, and 46.7% fewer memory
allocations on existing benchmarks.

          │ string_old.txt │           string_new2.txt           │
          │     sec/op     │   sec/op     vs base                │
String-16      3.622µ ± 5%   2.730µ ± 2%  -24.63% (p=0.000 n=10)

          │ string_old.txt │           string_new2.txt            │
          │      B/op      │     B/op      vs base                │
String-16     1.406Ki ± 0%   1.148Ki ± 0%  -18.33% (p=0.000 n=10)

          │ string_old.txt │          string_new2.txt           │
          │   allocs/op    │ allocs/op   vs base                │
String-16       60.00 ± 0%   32.00 ± 0%  -46.67% (p=0.000 n=10)

Change-Id: I70199be952eddc44134945077e52740e8921088f
Reviewed-on: https://go-review.googlesource.com/c/go/+/581155
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Joedian Reid <joedian@google.com>