Update the State class

[jeremyfowers]

Closes #152 by making the State class a lot more flexible.

Design considerations:

1. The main intention here was to eliminate the breaking changes and crashes that could result from making simple changes to the State class.
2. It turned out to be nice to keep a State class after all, since it can provide self-initialization and helper methods that a Dict would not.
3. It turns out we cant get rid of state.yaml entirely because we still need it for assessing cache hits/misses.

Big changes:

• State has been flattened and generalized.
  • There are far fewer hardcoded members.
  • Developers can add new members to State by assigning them as attributes, ie state.new_member = value.
  • There is no more Config class (e.g., build_name is now in state.build_name instead of state.config.build_name.
  • State is no longer a dataclass
• State is self-initializing and no longer relies on external functions in ignition to set its defaults.

Smaller changes:

• Most Enums have been converted to regular classes.

  • This allows us to safely save and load their values from YAML without worrying about converting to and from the Enum class.
  • Lots of .value statements have been removed.

• State.results is no longer a list. It simply contains whatever result a Stage wants to produce. Code should stop referencing results[0] or setting results = [output].

• State has been moved to common.filesystem to avoid circular imports.

  • Unified import of filesystem to fs in all modules (some did import ... as filesystem, now they are all import ... as fs)
  • Results in lots of changed of import build to import filesystem as fs

[ramkrishna2910]

Added some minor comment!
This PR greatly simplifies the code :)

[danielholanda]

Looking good!