v0.6.0

This is a major release with some important consistency and completeness
improvements. It should be treated as almost being the first release candidate
of 1.0.0, which will land after some small amount of further feature extensions
and documentation improvements.

Breaking changes:

Although there are some internal changes to API, as documented below, there are
no breaking changes to user-facing APIs.

New features and enhancements:

• Formula terms are now consistently ordered regardless of providence (formulae or
  manual term specification), and sorted according to R conventions by default
  rather than lexically. This can be changed using the _ordering keyword to
  the Formula constructor.
• Greater compatibility with R and patsy formulae:
  • for patsy: added standardize, Q and treatment contrasts shims.
  • for patsy: added cluster_by='numerical_factors option to ModelSpec to enable
    patsy style clustering of output columns by involved numerical factors.
  • for R: added support for exponentiation with ^ and %in%.
• Diff and Helmert contrast codings gained support for additional variants.
• Greatly improved the performance of generating sparse dummy encodings when
  there are many categories. #110 #112 (thanks @dbalabka)
• Context scoping operators (like paretheses) are now tokenized as their own special
  type.
• Add support for merging Structured instances, and use this functionality during
  AST evaluation where relevant.
• ModelSpec.term_indices is now a list rather than a tuple, to allow direct use when
  indexing pandas and numpy model matrices.
• Add official support for Python 3.11.

Bugfixes and cleanups:

• Fix parsing formulae starting with a parenthesis.
• Fix iteration over root nodes of Structured instances for non-sequential iterable values.
• Bump testing versions and fix poly unit tests.
• Fix use of deprecated automatic casting of factors to numpy arrays during dense
  column evaluation in PandasMaterializer. #122 (thanks @effigies)
• Factor.EvalMethod.UNKNOWN was removed, defaulting instead to LOOKUP.
• Remove sympy version constraint now that a bug has been fixed upstream.

Documentation:

• Substantial updates to documentation, which is now mostly complete for end-user
  use-cases. Developer and API docs are still pending.