Add support for function materialization.

 - Add `select_function` to `Platform` and implement it for `GenericCpu`
 - Add `SelectFunctions` AST pass

Squashed commit of the following:

commit e6b3aa856d2471b645f39d20673eb5259f7b1e1c
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Wed Mar 27 17:15:55 2024 +0100

    add select_function stub to GenericGpu

commit 679428f053d6bfd3a5ba4d1d1ff6c903577e9ed3
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Wed Mar 27 17:14:16 2024 +0100

    Remove IntrinsicsError in favor of MaterializaitonError. Move the latter to `backend.exceptions`.

commit 1a01cfde2f39b3394935c2a313312097d4cf66e0
Merge: d73d24c 0e4677de
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Wed Mar 27 17:07:31 2024 +0100

    Merge branch 'backend-rework' into b_function_mat

commit d73d24cf038950f2129e717a43dd09df11d725c0
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Tue Mar 26 18:19:08 2024 +0100

    introduce SelectFunctions and select_function protocol

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