Improve cast warnings/errors logic

[AndrewShf]

Fixes #155.
Merge together with eisop-codespecs/daikon#2.

This PR aims at refining the typecasting logic in Checker-Framework.

1. some downcasts can be statically verified and should not issue a warning or error. Suppose we are casting a @top base variable to @bot derived type, both the pluggable types and java types are downcasting, and suppose derived class can only be annotated with @bot. we believe this kinds of downcasts can be statically verified in checkerframework. Case1: If base is really a derived instance and this instance could only be annotated with @bot, it's okay we cast from top to bot; Case2: if base is not a derived instance, and we can rely on javac or java-runtime to report a inconvertible types or classcastexception.
2. some typecasts which can't be verified should throw an error instead of a warning, like said in Casting incomparable types should raise an error instead of a warning #155.

Enumerate all the conditions:

Main logic:

method isUpcast(...) logic:

isSafeDowncast logic:

isSafeIncomparableCast() logic:

[sonatype-lift]

⚠️ 97 God Classes were detected by Lift in this project. Visit the Lift web console for more details.

[AndrewShf]

checkCastElementType is mentioned in the manual and linked to two other places which doesn't mention the option at all.
should file an issue to add a new subsection to illustrate the typecasting and the option. #575

[wmdietl]

Thanks for all your work on this! I have a few questions about naming and how to structure this change.

[wmdietl]

This method seems very dangerous... can you add a warning to the method. I think this only works if the TypeVariables are from the same context. If one compares TVs from e.g. a declaration to a TV from a use site, there could be two different Ts that just happen to have the same name.

[AndrewShf]

Thanks for pointing this out! I will leave a comment there.

[wmdietl]

Further updates in #1049.