Restore support for alternate character sets

Needed by the execrable ncurses. Adds an extra branch in the hot path,
sigh. Thanks to branch prediction it doesnt have any measurable impact
on the benchmark, thankfully.

kitty/charsets.h

@@ -12,3 +12,4 @@
 uint32_t decode_utf8(uint32_t*, uint32_t*, uint8_t byte);
 size_t decode_utf8_string(const char *src, size_t sz, uint32_t *dest);
 unsigned int encode_utf8(uint32_t ch, char* dest);
+uint32_t* translation_table(uint32_t which);