Use stabilized Layout::repeat

[chescock]

Objective

BlobArray has copies of Layout::repeat with a note to replace them when it stabilizes. Layout::repeat stabilized in Rust 1.95, so let's use it!

Fix some UB in an edge case. BlobArray assumes that the size() of a type is a multiple of its align(). That's true for all Rust types, but a user may register a component using register_component_with_descriptor passing a Layout where it's not true. In release mode, that causes UB due to unaligned memory access. In debug mode, it triggers a debug_assert!, and then Table::alloc_columns aborts the process with AbortOnPanic.

Solution

Ensure that the Layout used by BlobArray has a size() that is a multiple of its align(), by making it a safety requirement of BlobArray and having ComponentDescriptor enforce it.

Remove the repeat_layout methods. Replace calls to array_layout with Layout::repeat_packed. Since size() is a multiple of align(), that has the same behavior as Layout::repeat, but is simpler. Replace calls to array_layout_unchecked with Layout::repeat_packed followed by debug_checked_unwrap.

[Victoronz]

Makes sense!

My understanding of the reason for Layout not enforcing this property on its own is that Rust types have the requirement, but allocations do not.
I think in practice however, most allocators already comply anyway, and we may one day get some updated Allocator trait with a version of Layout that properly enshrines this.