Investigate reducing the size of `MastNode` to 64 bytes

[plafer]

Originally posted by @bobbinth in https://github.com/0xPolygonMiden/miden-vm/pull/1466#discussion_r1744348232

Using vectors here increases the size of this struct from 40 bytes to 88 bytes. This also means that the size of MastNode enum grows to 96 bytes. Our goal should be to reduce the size of the MastNode to 64 bytes or fewer - but we can probably do it in a separate PR.

This might involve introducing the DecoratorSpan idea mentioned in https://github.com/0xPolygonMiden/miden-vm/issues/1122#issuecomment-2274957299.