The `uchr()` function takes a series of code point values and turns them
into an UTF-8 encoded string. The resulting string will have as many
characters as there were arguments to the function. The number of bytes
per character varies between 1 to 4, depending on the code point value.

Invalid numeric arguments or arguments being out of range 0-0x10FFFF will
be encoded as the Unicode replacement character 0xFFFD.

Returns the resulting UTF-8 string.

-- Testcase --
{{ uchr(0x2600, 0x2601, 0x2602) }}
{{ uchr("inval", -1, 0xffffffff) }}
-- End --

-- Expect stdout --
☀☁☂
���
-- End --