Converts an 8-bit to 64-bit unsigned value to a little-endian byte representation of the given size. If the size is not big enough to represent the value, then the high-order bytes are truncated.

Arguments:

• n: The value to convert.
• size: The size of the value, in bytes. This must be 8 or less, or task failure occurs. If this is less than 8, then a value of that many bytes is produced. For example, if size is 4, then a 32-bit byte representation is produced.
• f: A callback that receives the value.

This function returns the value returned by the callback, for convenience.