Moving things to Base Julia

We've been informally talking about moving stuff to Base. I figured we should actually have a record of what has been attempted and things we think are stable enough to consider.

• ArrayInterfaceCore.parent_type: https://github.com/JuliaLang/julia/pull/42923
• Non-mutating methods (deleteat, insert): https://github.com/JuliaLang/julia/pull/46453
• Static size related stuff currently all depends on either accepting https://github.com/JuliaLang/julia/pull/44538 or someone figuring out a functional alternative. A lot of methods depend on it (ArrayInterface.size, ArrayInterface.length ArrayInterface.strides, ArrayInterface.offsets, and all the known_* counterparts)
• ArrayInterfaceCore.can_change_size: https://github.com/JuliaLang/julia/pull/46500
• ArrayInterfaceCore.can_setindex: https://github.com/JuliaLang/julia/pull/46500
• ArrayInterfaceCore.isstructured
• ArrayInterfaceCore.has_sparsestruct
• ArrayInterfaceCore.issingular

Review the checklist in issue #340 and follow the linked Julia pull requests for the proposed Base changes. Start by determining which items are stable and which depend on PR #44538 or another functional alternative. Done means the candidate list and dependency status are resolved, but the issue does not define a self-contained implementation.