implement CidFromReader

And reuse two CidFromBytes tests for it, which includes both CIDv0 and
CIDv1 cases as inputs, as well as some inputs that should error.

Fixes #126.

cid.go

@@ -793,7 +793,7 @@ func CidFromReader(r io.Reader) (int, Cid, error) {
 	if cidLength > cap(br.dst) {
 		// If the multihash digest doesn't fit in our initial 64 bytes,
 		// efficiently extend the slice via append+make.
-		br.dst = append(br.dst, make([]byte, cidLength-len(br.dst))...)
+		br.dst = append(br.dst, make([]byte, cidLength-cap(br.dst))...)
 	} else {
 		// The multihash digest fits inside our buffer,
 		// so just extend its capacity.

cid_test.go

@@ -667,7 +667,6 @@ func TestReadCidsFromBuffer(t *testing.T) {
 		"k2cwueckqkibutvhkr4p2ln2pjcaxaakpd9db0e7j7ax1lxhhxy3ekpv",
 		"Qmf5Qzp6nGBku7CEn2UQx4mgN8TW69YUok36DrGa6NN893",
 		"zb2rhZi1JR4eNc2jBGaRYJKYM8JEB4ovenym8L1CmFsRAytkz",
-		"bafkqarjpmzuwyzltorxxezjpkvcfgqkfjfbfcvslivje2vchkzdu6rckjjcfgtkolaze6mssjqzeyn2ekrcfatkjku2vowseky3fswkfkm2deqkrju3e2",
 	}
 
 	var cids []Cid