Facilitate Hot-Restarts

[dhleong]

I've done some experiments with extracting and re-inflating file descriptors to do this the "traditional" way, but rebuilding the wrappers around the FD is a bit tricky in Java land (mostly due to how many layers our current stack builds). That's still the "safest" way to load in significant changes, but perhaps we can facilitate something like this (via tools.namespace), and maybe reloaded.repl. If we can safely reload all namespaces in the right order from the repl, that'd certainly be the fastest way to do it....