feat: `check_branch_node` and `check_extension_node` should check that embedded nodes have valid size

## Why
In a valid MPT, embedded nodes are such that their RLP encoding's length is strictly less than 32.

## What
Add a check in `check_branch_node` and `check_extension_node` such if a subnode is embedded, then its RLP encoding size must be less than 32.

## How
For each embedded subnode, RLP encode it and assert the resulting byte len is less than 32.