A number of small code quality improvements

[jschneider-bensch]

This PR adds more documentation to a bunch of smaller functions and renames some variables and functions to make naming more consistent and hopefully improve code legibility.

In addition it includes two changes that are meant to make the code more idiomatic:

• Directly include the u8 encoding in the HandshakeType enum. Similar treatment could be applied to other u8-encoding enums in the future, e.g. AlertDescription.
• Implement functions related to HandshakeData directly on the struct.

[franziskuskiefer]

Thanks! Making the code better one step at a time.
Only a few nits.

[franziskuskiefer]

Thanks