[reconfigurator] improvements to blueprint displays and tests

[sunshowers]

Bundles up a few changes:

1. Moves fmt::Debug for Blueprint to a display method. fmt::Debug output should ideally look close enough to standard fmt::Debug output, so moving this to some kind of fmt::Display is good.

   But why move this to its own struct? Two reasons:

   1. It can be somewhat surprising to get a massive multi-line output just by printing a fmt::Display impl. Using display makes this explicit.
   2. In the future, if we want more output customization (e.g. colorized output), the struct gives us a place to hang that customization off of.

2. By the same logic as above, moves diff display to its own struct as well.

3. Adds snapshot tests to ensure that changes to display output aren't surprising.

4. To achieve this, adds determinism to each test by using a tree of RNGs. Initially these RNGs are seeded from entropy, but we can also choose to seed them from any fixed hashable value.

   This tree of RNG approach is pretty standard in things like roguelikes, where I first came across it. It works quite well.