[model generation] Handle bank transfers outside of the pure functions

## Current state

Pure functions can add a `BankMsg` in order to signal funds transfer that result from a particular action being executed.
But when receiving funds, they are expected to transform the `bank` argument themselves.

## Proposed solution

Handle bank transfers outside of the pure functions (before/after) they are called, as @bugarela suggested.
They do not even need to read the bank state variable.