Reorder checks again

[loredanacirstea]

Fixes the issue from this thread: #47 (comment) stemmed from the fact that the code is less readable when the checks do not follow the implementation logic, but are artificially grouped together.

the beginning of a function starts with the requirements, first validating/requiring the value inputs, then the code is followed by the pure computation, which is followed by the validation of the computation and side-effects, and the code finishes with the asserts/invariants that must be true after the execution of the code.

• check function inputs
• check other preconditions to the state changes that will be done
• perform actions and immediately check if these actions succeeded
• check all other invariants at the end