This is a breaking API change that changes how wildcard patterns are
treated. In particular, wildcards are no longer allowed to appear at
arbitrary places in the URL, and are only allowed to appear immediately
after a path separator. This change effectively changes the wildcard
sigil from "*" to "/*".

Users who use wildcard routes like "/hello*" will have to switch to
regular expression based routes to preserve the old semantics.

The motivation for this change is that it allows the router to publish a
special "tail" key which represents the unmatched portion of the URL.
This is placed into URLParams under the key "*", and includes a leading
"/" to make it easier to write sub-routers.

This fixes the documentation wart where half of the methods were defined
on "rt *Mux". It doesn't change the public API.

The "dryrun" parameter on Pattern.Match was kind of ugly and made for an
exceedingly mediocre public interface. Instead, split its functionality
in two: the previous "dryrun" behavior now lives in the Match method,
and Patterns now actually mutate state when Run is called.

The code on the backend is of course still the same (for now), but at
least the interface is a little nicer.

Previously, the middleware stack passed the router a C, but this was
both odd semantically (a pattern which mutated the environment might see
a *different* environment) and bad for perf: it cost us an allocation.

Now we only pass around *C's internally.

Importantly ("importantly"), this gets us down to 0 allocations for the
static routing case, and one allocation (the URLParams map) for the
normal routing case.

I'm about to make changes to the way routes are selected, so we may as
well get good test coverage of that.

This bugfix brought to you by `go vet`

This change replaces a bit of API surface area (the Sub() method on Muxes) with
a slightly more expressive pattern syntax. I'm mostly doing this because it
seems cleaner: the "*" gets to take on a meaning very similar to what it means
in Sinatra (without growing regexp-like middle-of-a-path globbing, which sounds
terrifying and not particularly useful), and we get to nuke a useless function
from the API.

These were introduced by breaking changes in the following commits:
- aca58b22cf (ValidMethodsKey)
- 386323a6ed (HTTP methods are case sensitive)

Package web will now add a key to the environment when it fails to find a
valid route for the requested method, but when valid routes exist for other
methods.

This allows either the 404 handler or a sufficiently clever middleware layer to
provide support for OPTIONS automatically.