The spec imports are a mess to work with, so this branch cleans them up
a bit to ensure that we avoid generic sandwitches and that importing
stuff generally becomes easier.
* reexport crypto/digest/presets because these are part of the public
symbol set of the rest of the spec types
* don't export `merge` types from `base` - this causes circular deps
* fix circular deps in `ssz/spec_types` - this is the first step in
disentangling ssz from spec
* be explicit about phase0 vs altair - longer term, `altair` will become
the "natural" type set, then merge and so on, so no point in giving
`phase0` special preferential treatment
Era files contain 8192 blocks and a state corresponding to the length of
the array holding block roots in the state, meaning that each block is
verifiable using the pubkeys and block roots from the state. Of course,
one would need to know the root of the state as well, which is available
in the first block of the _next_ file - or known from outside.
This PR also adds an implementation to write e2s, e2i and era files, as
well as a python script to inspect them.
All in all, the format is very similar to what goes on in the network
requests meaning it can trivially serve as a backing format for serving
said requests.
Mainnet, up to the first 671k slots, take up 3.5gb - in each era file,
the BeaconState contributes about 9mb at current validator set sizes, up
from ~3mb in the early blocks, for a grand total of ~558mb for the 82 eras
tested - this overhead could potentially be calculated but one would lose
the ability to verify individual blocks (eras could still be verified using
historical roots).
```
-rw-rw-r--. 1 arnetheduck arnetheduck 16 5 mar 11.47 ethereum2-mainnet-00000000-00000001.e2i
-rw-rw-r--. 1 arnetheduck arnetheduck 1,8M 5 mar 11.47 ethereum2-mainnet-00000000-00000001.e2s
-rw-rw-r--. 1 arnetheduck arnetheduck 65K 5 mar 11.47 ethereum2-mainnet-00000001-00000001.e2i
-rw-rw-r--. 1 arnetheduck arnetheduck 18M 5 mar 11.47 ethereum2-mainnet-00000001-00000001.e2s
...
-rw-rw-r--. 1 arnetheduck arnetheduck 65K 5 mar 11.52 ethereum2-mainnet-00000051-00000001.e2i
-rw-rw-r--. 1 arnetheduck arnetheduck 68M 5 mar 11.52 ethereum2-mainnet-00000051-00000001.e2s
-rw-rw-r--. 1 arnetheduck arnetheduck 61K 5 mar 11.11 ethereum2-mainnet-00000052-00000001.e2i
-rw-rw-r--. 1 arnetheduck arnetheduck 62M 5 mar 11.11 ethereum2-mainnet-00000052-00000001.e2s
```
Jacek Sieka