v0.5.0

This is a major new release with some minor API changes, some ergonomic
improvements, and a few bug fixes.

Breaking changes:

• Accessing named substructures of Formula objects (e.g. formula.lhs) no
  longer returns a list of terms; but rather a Formula object, so that the
  helper methods can remain accessible. You can access the raw terms by
  iterating over the formula (list(formula)) or looking up the root node
  (formula.root).

New features and improvements:

• The ModelSpec object is now the source of truth in all ModelMatrix
  generations, and can be constructed directly from any supported specification
  using ModelSpec.from_spec(...). Supported specifications include formula
  strings, parsed formulae, model matrices and prior model specs.
• The .get_model_matrix() helper methods across Formula,
  FormulaMaterializer, ModelSpec and model_matrix objects/helpers
  functions are now consistent, and all use ModelSpec directly under the hood.
• When accessing substructures of Formula objects (e.g. formula.lhs), the
  term lists will be wrapped as trivial Formula instances rather than returned
  as raw lists (so that the helper methods like .get_model_matrix() can still
  be used).
• FormulaSpec is now exported from the top-level module.

Bugfixes and cleanups:

• Fixed ModelSpec specifications being overriden by default arguments to
  FormulaMaterializer.get_model_matrix.
• Structured._flatten() now correctly flattens unnamed substructures.