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

When the missing variable was not defined, we would get an exception
as a string doesn't contain provenance information.

- Add a test to prevent regression

This makes the 'Node' API public, and available for use directly for
plugins.

This replaces the helper method by adding a 'get_provenance' on the node
directly

- Adapt all call sites
- Delay getting provenance wherever possible without major refactor

- Implement __setitem__ on 'MappingNode'
- Implement __setitem__ on 'SequenceNode'
- Adapt all call sites to use the new calling way.

This mimics the dict.keys() method but returns a list instead of a
dict_keys, for cython performance reasons

- 'as_bool()' casts a ScalarNode into a boolean, understanding both
  'True' and 'False' as truthy-falsy values, as per node_get(type=bool)
  behavior
- 'is_none()' allwos checking whether the scalar node contains a 'None'
  value. Since 'None' cannot be used when working with booleans, we need
  to have a way of checking for 'None' when we actually need the
  information of whether the value is unset.
- Adapt all call places to use the new API

- 'get_scalar()' allows retrieving a scalar node from a mapping.
- 'as_str()' casts a ScalarNode into a string (thus removing the node
  information).

Both together, those replace 'node_get(mapping, key, type=str)' but
also allow retrieving the 'Node' itself, which will allow in the future
lazier provenance computation.

This requires the addition of a definition file (.pxd), to list
symbols exported.