Example: hoisting the condition `if buf[i] < 0x80 ...` out of DecodeVarint into DecodeTagAndWireType speeds up the included microbenchmark by over 10% on my mac

Example: hoisting the condition `if buf[i] < 0x80 ...` out of DecodeVarint into DecodeTagAndWireType speeds up the included microbenchmark by over 10% on my machine, just because it eliminates the function call for tags 1-15 (these are the magical tags, if you are using protobufs in a performance-sensitive context).