GSUnicode: implement non-lossy ASCII conversion - #127
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Both directions of the kCFStringEncodingNonLossyASCII codec were empty stubs. Implement them in GSUnicodeFromEncoding and GSUnicodeToEncoding: characters above printable ASCII become octal escapes (backslash then three digits) up to U+00FF and four-digit lowercase hexadecimal escapes (backslash u) above that, and a backslash doubles. Remove the two older standalone helpers, which were never wired in and did not work (the encoder never advanced its source pointer and used uppercase hexadecimal).
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The kCFStringEncodingNonLossyASCII codec was an empty stub in both GSUnicodeFromEncoding and GSUnicodeToEncoding, so CFStringGetBytes and CFStringCreateWithBytes produced nothing for it.
Implement both directions:
Encoding writes a character below 0x80 literally, doubles a backslash, writes characters up to U+00FF as a backslash followed by three octal digits, and writes higher characters as a backslash, a u and four lowercase hexadecimal digits.
Decoding reverses this, accepting octal escapes, backslash-u hexadecimal escapes (either case) and doubled backslashes, and copying other bytes through.
The two older standalone helpers GSUnicodeFromNonLossyASCII and GSUnicodeToNonLossyASCII are removed: they were never referenced, and did not work (the encoder never advanced its source pointer, an infinite loop, and produced uppercase hexadecimal; the decoder read past the end of its input).
Tests/CFString/nonlossy.m covers the octal range boundaries, the hex range, backslash doubling, a mixed string and round-tripping including uppercase-hex decode, verified against Apple CoreFoundation.