Fix conversion of GB18030 text (and add test suite)

- Truncated multi-byte characters are treated as an error
- Reject GB18030 4-byte codes which translate to (non-existent)
Unicod

Fix conversion of GB18030 text (and add test suite)

- Truncated multi-byte characters are treated as an error
- Reject GB18030 4-byte codes which translate to (non-existent)
Unicode codepoints above 0x10FFFF
- Add a number of missing mappings from the GB18030 standards
(These mappings are supported by iconv. I don't know why they were
missing from mbstring.)

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