Fix spelling

[laquazi]

Fixed some spelling mistakes in comments/docs and internal code.
Also consider changing these:

• SysExIncompleted (to SysExIncomplete)
• SysExCompleted (to SysExComplete)
• NOTPERMITTED (to FORBIDDEN)
• Lengthed
  Not mistakes, but they sound rather unnatural.

[melanchall]

Hi @laquazi,

Thanks for the PR!

As for suggestions:

• Not permitted - used by official Core MIDI documentation so I think it's better to stick with it.
• Lengthed - I understand, super strange word, but... maybe you can give me a hint: how to say that something has a length? One word, adjective is preferred. Also this word unfortunately is pretty old within the library and thus renaming will cause breaking changes :(

As for SysExIncompleted/SysExCompleted, thank you, I'll fix them.