Define main resource types

Functions, HTTPTriggers, Metadata, Environment.  Update the router to
use fission.Metadata, not fission.Function, to identify functions.

types.go

@@ -17,13 +17,41 @@ limitations under the License.
 package fission
 
 type (
-	Function struct {
+	// Metadata is used as the general identifier for all kinds of
+	// resources managed by the controller.  In general, when a
+	// resource is updated, the Name remains the same, but the UID
+	// changes.  In other words, the UID identifies a particular
+	// value of a resource.
+	Metadata struct {
 		Name string
 		Uid  string
 	}
 
+	// Function is a unit of executable code.  Though it's called
+	// a function, the code may have more than one function; it's
+	// usually some sort of module or package.
+	Function struct {
+		Metadata
+		Environment Metadata
+		Code        string
+	}
+
+	// Environment identifies the language and OS specific
+	// resources that a function depends on.  For now this
+	// includes only the function run container image.  Later,
+	// this will also include build containers, as well as support
+	// tools like debuggers, profilers, etc.
+	Environment struct {
+		Metadata
+		RunContainerImageUrl string
+	}
+
+	// HTTPTrigger maps URL patterns to functions.  Function.UID
+	// is optional; if absent, the latest version of the function
+	// will automatically be selected.
 	HTTPTrigger struct {
+		Metadata
 		UrlPattern string
-		Function
+		Function   Metadata
 	}
 )