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.