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.
