Use monad API instead of overloads

I noticed that at some places there are snippets such as this:
```
Overload monad_bind[local] = ``st_ex_bind``
Overload monad_unitbind[local] = ``\x y. st_ex_bind x (\z. y)``
Overload monad_ignore_bind[local] = ``\x y. st_ex_bind x (\z. y)``
Overload return[local] = ``st_ex_return``
```
The example here is from `candle/standard/monadic/holKernelScript.sml`.

It would be cleaner if instead the state-exception monad would be declared and (temporarily) enabled as a monad through HOL's `monadsyntax` API.

Ideally this should be transformed everywhere, meaning that no more overloads to `monad_bind` friends remain.