Update TreeHash to spec - add necessary padding

[mjkeating]

Please provide a brief description of the issue.

Needs necessary padding as per ethereum/consensus-specs#538

Describe the present behaviour of the application, with regards to this
issue.

Doesn't have padding

Steps to resolve

1. Change current hash_tree_root to hash_tree_root_internal
2. Add new hash_tree_root that pads result of hash_tree_root_internal to 32 bytes if necessary
3. Add necessary padding within merkle_hash algorithm

See updated spec: https://github.com/ethereum/eth2.0-specs/blob/master/specs/simple-serialize.md#tree-hash

[mjkeating]

I've started working on this.

[paulhauner]

Moving to https://github.com/sigp/lighthouse-common

Apologies if the repo re-org has caused inconvenience! (See #197)