Updated Serializer to be more efficient

[egbertbouman]

This PR:

• Updates Serializer to make it more efficient
• Updates EZPackOverlay so that there is less string slicing and fewer lists are being created
• Renames the (un)packing functions that take a Serializable as an argument
• Removes pack(_multiple)/unpack(_multiple) functions
• Removes optional_format_list as its use is limited, and it's hurting unpacking performance. I've intentionally put this in a separate commit, so it's easy to undo in case we decide to keep it.

With the updated serializer, the time spent (un)serializing typically decreases by 20+%.
For instance, for intro-request/responses the decrease varies between 21-38%. Currently:

Serializing 1,000,000 intro-request packets: 5.6937067 s
Serializing 1,000,000 intro-response packets: 7.4839985 s
Unserializing 1,000,000 intro-request packets: 7.712827699999998 s
Unserializing 1,000,000 intro-response packets: 9.7632044 s

With this PR:

Serializing 1,000,000 intro-request packets: 4.4311537 s
Serializing 1,000,000 intro-response packets: 5.894732000000001 s
Unserializing 1,000,000 intro-request packets: 4.850328299999999 s
Unserializing 1,000,000 intro-response packets: 6.008891500000001 s

[qstokkink]

It seems we are experiencing technical difficulties: Failure: 'pylint is not installed'.

[egbertbouman]

@qstokkink Nice catch! I didn't even notice, considering the check still passed.

[qstokkink]

@egbertbouman I noticed a hanging parenthesis and I wondered why Pylint didn't catch it. Turns out, it wasn't running.

[qstokkink]

Our sincere apologies for this technical intermission, please rebase onto the latest master.

[egbertbouman]

@qstokkink Thanks, the pylint check is working now!

[qstokkink]

As this is hyper-critical functionality, I'll perform an extended check beyond the PR test suite (running through the documentation, periodic network health scripts, etc.) when this PR is done. The final review will take a bit longer than usual.

[qstokkink]

I have finished the first half of my review: please address these two minor points.

I will now move into the second half of this review and run this code through all of our documentation and scripts.

[qstokkink]

Thank you for your contribution, fixes and your patience. I ran through all of our scripts and documentation and all of it still works as expected.