Cleanup:

Cleanup:
1. Move invalid two-byte sequences after valid ones
and make their descriptions easier to understand.
2. Replace the wrong and confusing expression "middle byte"
with the correct term "start byte".
3. Add test lines for U+EFFFF and U+F0000.
4. Replace the unhelpful word "strange" with more descriptive terms.
Arguably, nothing about this (or maybe everything?) is strange.