Fixes writing to constants, adds readonly opendream pseudo-type, and makes var overrides respect their var's type

[Altoids1]

Fixes #738, fixes #702, and fixes #535.

Summary

This bugfix stuff comes with a refactor of DMObjectBuilder into the static class it probably should be.

As a treat, this PR also has a commit that amends Content.Test so that it actually prints the message of any unexpected runtime into the log file, instead of just declaring that there was a runtime. Should be helpful :)

Changelog

• const now works as an effective keyword, preventing writing of values, within both procs and object definitions.
• Assignments now consistently remember the type of what they're assigning to (for implicit new calls)
• opendream_compiletimereadonly is now a valid type marker. Allows for marking something as unwriteable, while also not requiring the value to be literally a const.
• Content.Test now logs the message of any unexpected runtime discovered during testing.
• Fixed statics being overridable in derived classes.
• Miscellaneous null-safety improvements!

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