Add example compilation test to CI

Changes:
  * Make codec build command now is prefixed with "build"
  * A new "test" command to make
  * Add a Makefile for examples

examples/Makefile

@@ -0,0 +1,8 @@
+examples := $(shell find * -maxdepth 0 -type d)
+examples := $(filter-out internal,$(examples))
+
+.PHONY: all $(examples)
+all: $(examples)
+
+$(examples):
+	cd $@ && go build