Optimize URI.encode_www_form/1 and RFC 3986 query encoding

[NelsonVides]

Split out of #15733 — the loop rewrite only, without the SWAR fast path, details in the commit message.

Benchmark results, OTP 29 / MacBook M4:

Input	encode_www_form/1	rfc3986 encoding
empty	63.9 → 6.3 · 10.15×	66.2 → 6.6 · 10.07×
safe, 6 B	191.4 → 96.4 · 1.98×	169.3 → 95.4 · 1.77×
safe, 70 B	1422 → 376 · 3.78×	1126 → 372 · 3.02×
mixed, 70 B	1866 → 556 · 3.36×	1594 → 622 · 2.56×
safe, 700 B	16187 → 3268 · 4.95×	11521 → 3275 · 3.52×
mixed, 700 B	19337 → 4938 · 3.92×	16170 → 5933 · 2.73×

Allocation is now flat regardless of input size: 1.74 KB -> 104 B at 70 bytes, 16.51 KB -> 104 B at 700 bytes, and 104 B -> 0 B for empty. Reductions 3.51 K -> 0.70 K (safe 700 B).

Benchmark, Claude generated:

Mix.install([{:benchee, "~> 1.5"}])

defmodule Old do
  ...
end

defmodule New do
  ...
end

safe_70 = String.duplicate("abcdefg", 10)
mixed_70 = String.duplicate("abc /?~", 10)

inputs = %{
  "empty" => "",
  "safe, 6 bytes" => "abcdef",
  "safe, 7 bytes" => "abcdefg",
  "safe, 70 bytes" => safe_70,
  "mixed, 70 bytes" => mixed_70,
  "safe, 700 bytes" => String.duplicate(safe_70, 10),
  "mixed, 700 bytes" => String.duplicate(mixed_70, 10)
}

options = [
  inputs: inputs,
  time: 3,
  warmup: 1,
  memory_time: 1,
  reduction_time: 1,
  print: [fast_warning: false]
]

IO.puts("\n=== encode_www_form/1 ===\n")

Benchee.run(
  %{
    "old (comprehension)" => &Old.encode_www_form/1,
    "new (tail-recursive loop)" => &New.encode_www_form/1
  },
  options
)

IO.puts("\n=== rfc3986 unreserved encoding (per key/value in encode_query/2) ===\n")

Benchee.run(
  %{
    "old (comprehension)" => &Old.encode_rfc3986/1,
    "new (tail-recursive loop)" => &New.encode_rfc3986/1
  },
  options
)

[josevalim]

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