This adds a method to ensure that a value is from a set of valid values
and raises an error message accordingly.

- Define Enum types for each of the relevant cases
- Adapt call places that were doing such things manually

Make ScalarNode, MappingNode, and SequenceNode pickable by implementing
`__reduce__` for them.

All of the errors which subclass from BstError have their first
positional argument as message, LoadError should follow this
ordering for consistency.

`mapping.get_sequence(...).as_str_list()` is a very common
pattern seen both in plugins and the core. Adding a helper to reduce
the number of operations will make usage smoother

_create_new_file did not use the 'toplevel' parameter, we can therefore
remove it.

Users should not have to know anything about nodes, and should be
greeted by commonly defined yaml types.

A 'clone' operation has an implicit understanding that it is expensive,
which is not the case of a 'copy' operation, which is more usually
a shallow copy.

Therefore renaming to 'clone'

This reduces slightly the amount of C code needed and makes the run
marginally faster