`url` crate accounts for ~⅔ of a minimal worker binary — remove for next major

[guybedford]

A size analysis of the final optimized wasm output shows the url crate and its idna/ICU4X dependency stack dominate the binary of even a minimal worker:

Hello-world #[event(fetch)] worker	raw wasm	gzip
workers-rs today	231.9 KB	98.9 KB
with idna_adapter pinned to 1.0 (no ICU)	125.0 KB	53.5 KB
with url removed entirely	76.5 KB	32.5 KB

(Methodology: opt-level = "z", lto = true, codegen-units = 1, wasm-bindgen 0.2.127, wasm-opt -O per worker-build defaults, gzip -9. Router-based worker shows the same ~155 KB delta: 256.8 KB → 102.2 KB.)

That's ~107 KB of ICU4X Unicode tables (pulled in via idna since url 2.5) plus ~48 KB of parser/punycode code — to reimplement in wasm a WHATWG URL parser the runtime already provides natively.

Worse, every worker pays this even if it never names a Url: the From<web_sys::Request> conversion eagerly parses the URL of every incoming request just to precompute the path field, so the parser is always retained:

path: Url::parse(&req.url())
    .map(|u| u.path().into())

req.url() is produced by the runtime's own URL parser, so this is a redundant reparse — the path can be extracted with simple string slicing (verified: doing so strips all 95 url/idna/icu symbols from the binary).

Proposal:

• Minor (now): compute Request::path without Url::parse, making the url stack dead-code-eliminable for workers that never use Url. Document the idna_adapter = 1.0 pin for users who parse URLs but don't need non-ASCII IDNA.
• Major (next): remove url::Url from the public API (Request::url, Fetch::Url, RequestInit, router) in favor of a platform-backed URL type wrapping the runtime's native parser, eliminating the dependency entirely.

Related: #380, #1033 (re-exports that further entrench the url public API surface).