net/url: reduce allocs in Encode

[jub0bs]

This change adds benchmarks for Encode and reverts what CL 617356 did in
this package. In this case, pre-sizing the slice in which we collect the
keys and then sorting it is indeed more performant than relying on a
combination of maps.Keys and slices.Sorted.

Here are some benchmark results:

goos: darwin
goarch: amd64
pkg: net/url
cpu: Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
│ old │ new │
│ sec/op │ sec/op vs base │
EncodeQuery-8 1.697µ ± 1% 1.049µ ± 1% -38.17% (p=0.000 n=20)

          │    old     │                new                 │
          │    B/op    │    B/op     vs base                │

EncodeQuery-8 576.0 ± 0% 256.0 ± 0% -55.56% (p=0.000 n=20)

          │    old     │                new                 │
          │ allocs/op  │ allocs/op   vs base                │

EncodeQuery-8 25.00 ± 0% 11.00 ± 0% -56.00% (p=0.000 n=20)

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