This changes the internal stock strings API to be more strongly typed,
ensuring that the caller can not construct the stock string in the
wrong way.
The old approach left it to the callers to figure out how a stock
string should be created, now each stock string has their specific
arguments and callers can not make mistakes. In particular all the
subtleties and different ways of calling stock_system_msg() disappear.
This could not use a trait for stock strings, as this would not allow
having per-message typed arguments. So we needed a type per message
with a custom method, only by convention this method is .stock_str().
The type is a enum without variants to avoid allowing someone to
create the type.
Sadly the fallback string and substitutions are still far away from
each other, but it is now only one place which needs to know how to
construct the string instead of many.
- doc fixes
- make BobStateHandle safer by moving the state out of the handle.
- handle more match cases explicit in BobState returns
- fewer mutable variables
This does not only organise things better, but most importantly the
BobStateHandle is now not in the same module as its users. This means
it can guarantee safety about how it is initialised and use
unreachable!() to simplify it's API.
