Use a custom buffer intead of buffers from bytes

This commit implements a new buffer that eliminates a few copies.

buffer.go

@@ -0,0 +1,42 @@
+package cmux
+
+import "io"
+
+type buffer struct {
+	read int
+	data []byte
+}
+
+func (b *buffer) Read(p []byte) (n int, err error) {
+	n = len(b.data) - b.read
+	if n == 0 {
+		return 0, io.EOF
+	}
+
+	if len(p) < n {
+		n = len(p)
+	}
+
+	copy(p[:n], b.data[b.read:b.read+n])
+	b.read += n
+	return
+}
+
+func (b *buffer) Len() int {
+	return len(b.data) - b.read
+}
+
+func (b *buffer) resetRead() {
+	b.read = 0
+}
+
+func (b *buffer) Write(p []byte) (n int, err error) {
+	n = len(p)
+	if b.data == nil {
+		b.data = p[:n:n]
+		return
+	}
+
+	b.data = append(b.data, p...)
+	return
+}