Allow lookup of code from name

[willscott]

currently, one can only look up the string name from a multicodec code. if you have a string code and want to know the multicodec number for it, there is no easy way to do so with the current library.

[willscott]

e.g. it leads to partial re-implementations of this table in e.g. https://github.com/ipfs/go-ipfs/blob/master/core/coredag/dagtransl.go#L20-L53