Refactor Type Handling and Typification

 - Add a `dtype` member to all expression nodes
 - Make the `Typifier` apply `dtype`s to all expressions
 - Adapt transformations and IterationSpace to set data types on created
   expressions
 - Refactor TypeContext and contextual typing interface to be more
   intuitive
 - Refactor the Typifier to apply more operations through the
   TypeContext

Squashed commit of the following:

commit 3e81188a318aa1dc294cf0cd11bf2ec7f62a9b55
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Wed Mar 27 17:00:17 2024 +0100

    Improve typification of integer expressions

     - Check integer type constraint in `_apply_target_type` to correctly catch deferred expressions

commit 63d0cfa5ea1b8a41c9a74bbfcf0618fad03ffa48
Merge: 671f057 075ae357
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Wed Mar 27 16:46:28 2024 +0100

    Merge branch 'backend-rework' into b_refactor_typing

commit 671f0578a39e452504243019dab28d93f0114082
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Tue Mar 26 16:39:43 2024 +0100

    Fix documentation for Typifier and PsExpression

commit 3ec258517ad8a510118265184b5dc7805128dcd3
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Mon Mar 25 17:14:21 2024 +0100

    Typing refactor:

      - Annotate all expressions with types
      - Refactor Typifier for cleaner information flow and better
        readability
      - Have iteration space and transformers typify newly created AST nodes