Add fast path for simple URL parsing

This patch adds a fast path for parsing of simple path-only URLs, as commonly
found in HTTP requests received by a server.

Benchmark results [ms], before / after patch:
```
/foo/bar              0.008956   0.000418 (fast path used)
http://example.com/   0.011426   0.011437 (normal slow path, no change)
```

In a simple ab benchmark of a single-threaded web server, this patch
increases the request rate from around 6400 to 7400 req/s.