Transaction Leaf proof

[nikkolasg]

Add transaction leaf proof circuitS:

• first one decodes all headers sequentially
• second one takes directly an offset and extract gas value directly.

There are potentially still many optimization to be done on the first one which is the only one really secure.

[lopeetall]

Everything looks great, except I'm confused about some functions in the test module in utils.rs. If you can explain what they are for and why they are placed where they are I can approve :)