Get code point at code unit offset

Property definitions

core $ UCharString :: char_at_offset
	# Get code point at code unit `offset`
	fun char_at_offset(offset: Int, code_units: Int): Char `{
		UChar32 c = 0;
		U16_NEXT(self, offset, code_units, c);
		return c;
	`}
lib/core/text/u16_string.nit:181,2--186,3