Get rid of args setting

It's only used in the `restart` command and the `restart` command already
accepts arguments.
Remove Byebug.start options.

They are regular settings that should be set in .byebugrc just like the
rest of the settings.
Refactor (again) settings

One class per setting seems more flexible and readable to me
Minor whitespace and code style fixes
Remove `trace` command for line tracing.

`set linetrace` should be used instead. For tracing global variables,
`tracevar` command is provided.
Remove unnecessary stuff
Remove `show version` setting

Setting is stuff that can be set & show, version can't be set by user
Fix some load order issues
Fix autolist setting initialization
Fix some save tests
Properly skip tests
Whitespace
Fix last restart test
More test fixes
Fix autoeval setting