Add function to determine the size of final unixfs DAG given raw bytes

It would be fantastic to have a function that given the size of raw bytes would cheaply and quickly calculate the size of the final `unixfs` DAG. This would be useful in cases where the final DAG will be written out into a fixed sized bucket and the user wants to guarantee that each bucket will have the full raw bytes, e.g., FileCoin pieces of size 32GiB.

Other uses of this include pre-allocation of resources or more efficient buffer management depending on the final DAG size for large-scale data intensive applications.