Improve `mlar info`: tests and raw size output

[commial]

mlar info has been introduced in #40.

It can be improved by:

• Outputting the total payload size
• Testing the output with a Regexp to ensure the numbers are corrects (or at least, for the compression ratio, better than the total file size divided by the total payload size).

[commial]

Additionally, the command might report what layers are enabled, before requiring the private key.

Also, an output computer-readable would be nice (for instance, json or yaml)

[extiop]

As of today, the following points are considered relevant for this issue:

• Output the total payload size
• Layers (via magics):
  • Compression (since there is no layer magic, another method must be implemented)
  • Encryption
  • Signature
• Output in a computer-readable format: JSON -- with a specific flag to get that output from mlar info