Factor C2D out into a library for reuse

Hybrid State Feedback Loop already has an implementation.  Grab it.

Change-Id: I8b294e3ee7d82249cbd2b0c732086a735a57c36b
diff --git a/frc971/control_loops/BUILD b/frc971/control_loops/BUILD
index 7988e51..b6b0362 100644
--- a/frc971/control_loops/BUILD
+++ b/frc971/control_loops/BUILD
@@ -155,6 +155,7 @@
         "hybrid_state_feedback_loop.h",
     ],
     deps = [
+        ":c2d",
         ":state_feedback_loop",
         "//aos:macros",
         "//aos/controls:control_loop",
@@ -265,6 +266,17 @@
 )
 
 cc_library(
+    name = "c2d",
+    hdrs = [
+        "c2d.h",
+    ],
+    visibility = ["//visibility:public"],
+    deps = [
+        "//third_party/eigen",
+    ],
+)
+
+cc_library(
     name = "dlqr",
     hdrs = [
         "dlqr.h",