Explore using immutable objects for various hooks

[btford]

One of the problems with middleware is that it makes it hard to reason about where changes to request metadata come from.

This router will have the same problem with its "pipeline," which is effectively middleware.

I'd like to explore having a "dev mode," that enforces immutability with Object.freeze, that could be turned off in production for a perf boost.

[SanderElias]

I think this is a good idea. Perhaps even in production, unless the benchmarks show that this is really a big hurdle.
Too bad that es6 proxies have such a limited support, as those would be an ideal fit.