add caching in hashTreeRoot

[anshalshukla]

Adds caching in hashing path for List and BitList

• Supports caching only when hashing with Sha256
• init API doesn't take in Hasher as an input so CacheType cannot be Hasher agnostic without API changes
• Can support multiple Hasher in CacheType either by updating init API or by storing a list of CacheType which can be appended based on the Hasher call in hashTreeRoot