Add `@CheckReturnValue` (JSR-305) annotation to certain key methods

Add `@CheckReturnValue` (JSR-305) annotation to certain key methods, since these methods being called without further calling their fluent chain would result in falsely-passing tests.

This will result in Error-Prone or any other static analysis system flagging as an error any usage of `ASSERT.that(foo);`  This is especially important for the erroneous use of booleans such as `ASSERT.that(a == b);`