+ * Convert native (big-endian) UTF-16 to UTF-8. For reversed (little-endian),
+ * use utf16_to_utf8_reversed().
+ *
+ * UTF-16 requires 2 bytes for every code point below 0x10000; otherwise 4 bytes.
+ * UTF-8 requires 1-3 bytes for every code point below 0x1000; otherwise 4 bytes.
+ * UTF-EBCDIC requires 1-4 bytes for every code point below 0x1000; otherwise 4-5 bytes.
+ *
+ * These functions don't check for overflow. The worst case is every code
+ * point in the input is 2 bytes, and requires 4 bytes on output. (If the code
+ * is never going to run in EBCDIC, it is 2 bytes requiring 3 on output.) Therefore the
+ * destination must be pre-extended to 2 times the source length.