Platform resource reports

[whitequark]

nMigen platforms are much more powerful than typical constraint files: apart from pin locations they also specify pin polarities, support indirect addressing through connectors, and in some cases include multiple versions of the same pinout. In the end, it is not always clear what pin ended where and with what polarity.

We should add an API for retrieving the final pin mapping (including all intermediate connectors) and polarity, and include a human-readable report generated from this pin mapping in the build plan (aka the build directory).