This is a major breaking change to web.C, the Goji context object. The
Env key has been changed from a map[string]interface{} to a
map[interface{}]interface{} in order to better support package-private
environment values and to make the context value more compatible with
golang.org/x/net/context's Context.

Since strings support equality, most existing uses of web.C should
continue to function. Users who construct Env by hand (i.e., by calling
"make") will need to update their code as instructed by their compiler.
Users who iterate over the environment will need to update their code to
take into account the fact that keys may no longer be strings.

In particular, these started failing when running tests under the race
detector in Go 1.4 [0], probably due to some kind of (GC?) hijinks
clearing out the sync.Pool.

[0]: these tests might have also failed in 1.3, but I didn't check

This has the additional benefit of removing the need for a go1.3 branch.

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.

The "name" parameter was originally a workaround for the fact that function
pointers in Go never compare equal to each other (unless they are both nil).
Unfortunately, this presents a pretty terrible interface to the end programmer,
since they'll probably end up stuttering something like:

    mux.Use("MyMiddleware", MyMiddleware)

Luckily, I found a workaround for doing function pointer equality (it doesn't
even require "unsafe"!), so we can get rid of the name parameter for good.

My tests caught some bugs! Amazing!