Composer and language Frontend:

    - Minimialize exposition of `SfgVar`, use `AugExpr` in all composer
    interfaces instead
    - Move `SfgVar` into `lang` module
    - Introduce `VarLike` and `ExprLike` protocols to the `lang` module
    - Treat `TypedSymbol` equivalently to SfgVar in interfaces
    - Disallow `sp.Symbol` in expressions
    - Deprecate `map_param` in favor of newly introduced `set_param`,
    which uses `AugExpr`
    - Deprecate `sfg.define` in favor of `sfg.code`
    - Introduce `Ref` type

IR Postprocessing:

    - Check for type conflicts during live-variable collection

Documentation:

    - Add docstrings to `lang.expressions` module
    - Add various doctest examples to docstrings
    - Link to pystencils via intersphinx

Tests:

    - Fix bugs in generator script testing
    - Add new testscripts
    - Add unit tests for parts of `lang.expressions` and
    `ir.postprocessing`

Squashed commit of the following:

commit c75b939d
Aut...