Extend CircleBuffer and UART to support 16-bit buffer sizes.
Redesign Defer to allow a different event bus and event for each timer.
Modify DeferTable so that each 3-tuple of timer information is grouped
  in a single array. This makes it easier to support variable-length tables
  in the future.
Extend EventBus to notify a "parent" bus whenever a signal is raised in the
  child. This alows for a hierarchy of event busses.
Provide RawComm with its own EventBus subordinate to the application bus.
Refactor RawComm state into a struct, and provide an application-level hook
  to acquire the RawComm state from otherwise-contextless interrupts.
Add an (as yet unused) additional CircleBuffer to the UART driver.
Make UART event bus and signal configurable.
Implement a uitoab(), converting an unsigned int to a string for a given base.
Add preliminary SD card driver.