[minor] Move for- and while-loops to their own submodule

[liamhuber]

This is a pure refactor. It's a patch for users, since these are still accessed at Workflow.create.meta..., but it's minor for node developers since the constructors live in a different module.

The intent is to clear the meta.py namespace up a bit, as it looks like I can combine StaticNode with custom __reduce__ methods to get node class factories that produce pickle-able nodes for variable IO formats!

[github-actions[bot]]

:point_left: Launch a binder notebook on branch _pyiron/pyiron_workflow/moveloops

[codacy-production[bot]]

Coverage summary from Codacy

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Coverage variation	Diff coverage
:white_check_mark: +0.05% (target: -1.00%)	:white_check_mark: 97.62%

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (bef19589f52bb6db071097a071f6cc9f2dede4a3) | 3500 | 3066 | 87.60% | | | Head commit (f4fdaa702c5199b546f0f3726280c701eafcf34f) | 3507 (+7) | 3074 (+8) | 87.65% (**+0.05%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#291) | 42 | 41 | **97.62%** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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