* rename from tree_ssz
* add special cases for some types based on beacon chain code
* add smoke test
* avoids some trivial allocations, but the big one remains - the
temporary
chunk buffer is still heap allocated
* update to handle zero-length buffer case same as latest spec
* first part of spec updating; some constants, structure changes, data type changes from int to uint
* Update beacon_chain/datatypes.nim
Co-Authored-By: tersec <tersec@users.noreply.github.com>