confusing limit on unit size

[jba]

The spec says that a unit string "can have a maximum length of 63 characters. The number 63 is chosen to allow the unit strings (including the \0 terminator on certain language runtimes) to be stored and compared as 8-bytes integers when performance is critical."

You can only fit 8 (ASCII) characters into 8 bytes, so I'm not sure what the intent here is (eight 8-byte integers?), or if this is just a mistake.

Also, I see elsewhere (#705) that units should follow the UCUM standard, but I don't see any length restrictions there.