Use a continous []byte buffer as param for Hash function

[ferranbt]

The current Hash function takes as input and output parameters a slice of [32]byte objects which represent the list of objects to hash. The proposal is to replace that input with a bytes buffer []byte where all the objects are allocated continuously. This memory structure is easier to optimize and reuse in Go than the other one.

This is a fork made by @potuz with the changes and this is a PR on ferranbt/fastssz with the changes applied. The changes are minimal and the code gets 2x faster.