Rework error handling

[SwiftStudies]

The new stack and rule system enables far more simple error handling, but the code is still littered with old special cases that are no longer required.

Before release of v1 this should be cleaned up to use simple hierarchical errors and all error handling removed from IRs and parsing strategies.

[SwiftStudies]

Although there is more work to do, the fundamental architecture is now there, and the old cruft cut away. Fixed in #84