byte encoding for multiaddr

[DieMyst]

Multiaddr specification has convention about machine-readable format that use unsigned varint:
Machine-readable multiaddr: (<protoCode uvarint><value []byte>)+
Same example: 0x4 0x7f 0x0 0x0 0x1 0x91 0x2 0x4 0xd2
Values are usually length-prefixed with a uvarint

it is good to write codecs for this format.