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.

Change the per-process nonce part of the request ID from 8 characters to
10, and wrap the entire thing in a retry loop so you can never get an
"unlucky" panic. I know this will "never" happen in practice, but it
doesn't hurt to make sure we never, ever have any collisions, and never,
ever have any runtime panics.

It's also worth documenting the math ("math") I used to calculate the
numbers here.

I've currently written three:
- Request ID generation
- Request logging, with color (!)
- Recovery from panics