Fix conversion of CP936 text (and add test suite)

- Treat truncated multi-byte characters as an error.
- Don't allow ASCII control characters to appear in the middle of a
multi-byt

Fix conversion of CP936 text (and add test suite)

- Treat truncated multi-byte characters as an error.
- Don't allow ASCII control characters to appear in the middle of a
multi-byte character.
- Adjust some mappings to match recommendations in conversion table
from Unicode Consortium.

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