Linearise message length padding

[ghost]

To standardise the way that message length padding is done this patch makes it the same as the one I've submitted to the Conversations app. Hence any adversary only sees the same message lengths and can't infer anything from relative comparisons.

See https://github.com/siacs/Conversations/pull/1926

[codecov-io]

Current coverage is 69.00%

Merging #12 into master will not change coverage

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  Files            13         13          
  Lines           584        584          
  Methods           0          0          
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  Branches         46         46          
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  Hits            403        403          
  Misses          167        167          
  Partials         14         14          

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[ghost]

Any other thoughts on this? Good/bad?