perf: speed up URL parsing and IDNA ToASCII

[geeknoid]

Avoid redundant work on the URL parsing hot path:

• host: only percent-decode when the input contains a %, and append domain hosts to the serialization with push_str instead of write!.
• parser: skip the userinfo scan when the authority has no @, and advance the input iterator by the known byte length after scanning a host rather than re-iterating it character by character.
• idna: pass ASCII labels with a leading digit through the fast path when the domain cannot be a bidi domain name.

Add Callgrind (gungraun) instruction-count benchmarks alongside the existing wall-clock ones.

benchmark	instr orig	instr new	Δ%	ns orig	ns new	ns Δ%
plain	2403	1823	-24.1%	209	168	-19.6%
short	2653	2074	-21.8%	243	186	-23.5%
long	3517	2935	-16.5%	306	253	-17.3%
fragment	3887	3306	-14.9%	344	285	-17.2%
port	2965	2318	-21.8%	321	264	-17.8%
hyphen	3573	2607	-27.0%	307	235	-23.5%
leading_digit	3674	2315	-37.0%	315	211	-33.0%
unicode_mixed	6791	6128	-9.8%	729	678	-7.0%
punycode_mixed	6105	5289	-13.4%	607	498	-18.0%
unicode_ltr	8379	7609	-9.2%	847	788	-7.0%
punycode_ltr	8265	7249	-12.3%	793	692	-12.7%
unicode_rtl	8547	7853	-8.1%	834	766	-8.2%
punycode_rtl	8336	7306	-12.4%	780	660	-15.4%
url_to_file_path	2736	2734	-0.1%	285	280	-1.8%

[hsivonen]

Thanks. The IDNA change here may become moot with the redesign required by #1135.

[geeknoid]

@hsivonen Can we go ahead and merge this PR, it's no-regrets improvement across the board. If some of it is thrown out due to a change in the future, that won't be a problem, right?

[hsivonen]

I'm OK with landing the idna part. I defer to Manish or Valentin for the parts outside idna.

[codecov]

Codecov Report

❌ Patch coverage is 82.69231% with 9 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@25137be). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
idna/src/uts46.rs	78.26%	5 Missing ⚠️
url/src/parser.rs	81.25%	3 Missing ⚠️
url/src/host.rs	92.30%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1142   +/-   ##
=======================================
  Coverage        ?   86.94%           
=======================================
  Files           ?       26           
  Lines           ?     5315           
  Branches        ?        0           
=======================================
  Hits            ?     4621           
  Misses          ?      694           
  Partials        ?        0           

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[Manishearth]

Rest also seems fine

[Manishearth]

Not overly happy about adding another benchmarking framework but it's fine.

[Manishearth]

CI fails