feat(nix/buildTypedGo): Add a //nix/buildGo wrapper for typed Go

These functions work like buildGo.program & buildGo.package, but run
the .go2 sources through go2go first before passing them to the
//nix/buildGo equivalents.

Change-Id: Id1ebab6085c390d6986387370181377b9f5d39e8

nix/buildTypedGo/example/default.nix

@@ -0,0 +1,8 @@
+{ depot, ... }:
+
+depot.nix.buildTypedGo.program {
+  name = "example";
+  srcs = [
+    ./main.go2
+  ];
+}

nix/buildTypedGo/example/main.go2

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"fmt"
+)
+
+func Print(type T)(s []T) {
+	for _, v := range s {
+		fmt.Print(v)
+	}
+}
+
+func main() {
+	Print([]string{"Hello, ", "TVL\n"})
+}