Full restructure in accordance with

- pkg/{subsystem}/{package} style
  - modify Makefile to reflect the new style,
    consolidate various entries
  - add a dummy ``main.go`` at top level

pkg/storage/erasure/isal/README.md

@@ -0,0 +1,11 @@
+- Install 'build-essential'
+
+~~~
+  # apt-get install build-essential -y
+~~~
+
+- Install 'yasm'
+
+~~~
+  # apt-get install yasm -y
+~~~