Rollup merge of rust-lang#82726 - ssomers:btree_node_rearange, r=Mark…

…-Simulacrum

BTree: move blocks around in node.rs

Without changing any names or implementation, reorder some members:
- Move down the ones defined long ago on the demised `struct Root`, to below the definition of their current host `struct NodeRef`.
- Move up some defined on `struct NodeRef` that are interspersed with those defined on `struct Handle`.
- Move up the `correct_…` methods squeezed between the two flavours of `push`.
- Move the unchecked static downcasts (`cast_to_…`) after the upcasts (`forget_`) and the (weirdly named) dynamic downcasts (`force`).
r? `@Mark-Simulacrum`