internal/subtle: document and test XORBytes overlap rules

[emmansun]

XORBytes doesn't say anything about how it deals with destination and source overlaps. Current implementations as written do work if the destination overlaps perfectly with a source, but will unavoidably return nonsensical results if the destination is ahead of the source.
Lock in the current behavior with tests, docs, and panics.

Note that this introduces a new panic, but if any applications run intoit we are potentially catching a security issue.

[emmansun]

v0.29.3