* support downloading blocks / states via JSON in addition to SSZ -
slow, but needed for infura support - SSZ is still used when server
supports it
* use common forked block/state reader in REST API
* fix stack overflows in REST JSON decoder
* fix invalid serialization of `justification_bits` in
`/eth/v1/debug/beacon/states` and `/eth/v2/debug/beacon/states`
* fix REST client to use `/eth/...` instead of `/api/eth/...`, update
"default" urls to expose REST api via `/eth` as well as this is what the
standard says - `/api` was added early on based on an example "base url"
in the spec that has been removed since
* expose Nimbus REST extensions via `/nimbus` in addition to
`/api/nimbus` to stay consistent with `/eth`
* fix invalid state root when reading states via REST
* fix recursive imports in `spec/ssz_codec`
* remove usages of `serialization.useCustomSerialization` - fickle
0e2b4e39fa