introduce rewindable_kernel_view (readonly but rewindable kernel MMR view)

[antiochp]

• introduce rewindable_kernel_view
• break txhashset out into a module to keep files clean
• set a header on a txhashset extension
  • no need to pass one in on various calls to the ext now
  • update it on rewind() as necessary

We can now validate the full history of kernel roots via -

		let mut current = header.clone();
		txhashset::rewindable_kernel_view(&txhashset, |view| {
			while current.height > 0 {
				view.rewind(&current)?;
				view.validate_root()?;
				current = view.batch().get_block_header(&current.previous)?;
			}
			Ok(())
		})?;

All of this is to explore a kernel specific view rather than starting a new full txhashset extension. We only need the kernels when we validate the kernel roots back through history (and it needs to be rewindable, but does not need to be writeable).

There is limited value here when just thinking about the kernel MMR, but we are considering putting PoW in an MMR to implement Flyclient (see #1555) and it would be nice to have some flexibility in terms of how we access txhashset and the underlying MMRs.

I imagine something similar to this rewindable, readonly kernel view for the (yet to exist)
"BlockHeader MMR".