FR: Lazy section evaluation

[ershov]

Feature request

Add a -lazy flag to the section command so that if the section is collapsed and its size is known before it is parsed, the actual parsing is skipped until the section is expanded.

This would be very beneficial for big files with complex structure like archives or databases.

Alternatively, could just add a -size argument and do it automatically if it's accompanied with a -collapsed. An additional benefit is that it would also do an automatic error recovery in that section: in case of an error within the section, the outer structures will still be parsed properly.

TODO: Consider the use of state variables. In case if the code uses/changes some of them, that flag still should be explicit since it changes the behavior.