* cannon: Isolate test utilities, rework types to avoid circular dependencies

* cannon: Rollback changes to MIPSEVM.EncodePreimageOracleInput

* cannon: Move open_mips_tests to tests subpackage

* cannon: Update some paths to open_mips_tests

* cannon: Collect oracle utils, move generic tests to tests package

* cannon: Move common state constants and utilities to core pkg

* cannon: Move state.go and state_mt.go into impls package

* cannon: Create a memory subpackage

* cannon: Create exec pkg for mips logic

* cannon: Wrap remaining core files in isolated pkgs

* cannon: Create a few more subpackages

* cannon: Rename state_mt to state

* cannon: Move hex utility

* cannon: Pull loading functionality out of patch.go

* cannon: Remove 'MT' prefixes on multi_threaded/state.go symbols

* cannon: Add compile-time PreimageOracle check back in

* cannon: Undo excessive nesting in core package

* cannon: Reorganize single- and multithreaded pkgs

* cannon: Rename test_util pkg to testutil

* cannon: Move instrumented_state logic into singlethreaded pkg

* cannon: Fix test paths

* cannon: Move core pkgs back to root mipsevm pkg

* cannon: Flatten util pkg back into main mipsevm pkg

* cannon: Cleanup - don't use vmstate for mipsevm pkg

* cannon: Add godoc for FPVMState.GetRegisters()

* cannon: Cleanup - fix pkg name, fn visibility