Adopt Checker Framework for more readable primitives

[jflo]

In order to resolve a tension between performance and readability, we should adopt Checker Framework as discussed here:

#5536 (comment)

This will allow us to use longs as 64bits of storage, and still convey to the reader that their signedness must be ignored and that particular care is necessary when doing arithmetic operations with them.

We should also update developer docs if setting up checker framework as an annotation processor requires anything beyond the normal instructions.

• https://blog.rcook.org/blog/2019/01-intellij-checker-framework/
• https://checkerframework.org/