Lazy storage

Target to change back to proto-proposal once !317 (merged) is merged (gitlab is so bad at handling dependencies!)

Contains:

• !96 (merged)
• !274 (merged)
• !244 (merged)
• some commits or part of them from @clarus1, can't find the MR reference because we don't keep track of them
• remove Env change or add full signature of tzOption (done in !323 (merged))
• fix RPC in case there are several bigmaps
• add docstrings
• add some unit tests and make a list of other tests to do after this MR (follow up on #213) (some done in !341 (merged))

The goal of lazy storage is to generalize what is done for big maps in Michelson so that we can add more types with a similar behaviour without worrying too much.