Add export-history subcommand.

[henridf]

geth export-history exports blocks and receipts in SSZ format.

running todo:

• [x] Add logging
• [x] Check cli params: start < end
• [x] Print out hash root for info purposes (it can also be computed with bart )

[lightclient]

I rebased to master.

[henridf]

@lightclient I moved the ssz definitions into the repo.

Unfortunately right now annotating types.Block isn't possible due to some current limitations of fastssz:

• can't handle the common.Hash and common.Address types in Header (https://github.com/ferranbt/fastssz/issues/108 - I will take a stab at at).
• big.Ints aren't supported (https://github.com/ferranbt/fastssz/issues/107)

[holiman]

Superseded by #26621