Remove compat layer and use generics directly

Backwards compatibility is not required since moving to v2.

nanotime_time.go

@@ -0,0 +1,13 @@
+//go:build purego || appengine || js
+
+// This file contains the safe implementation of nanotime using time.Now().
+
+package puddle
+
+import (
+	"time"
+)
+
+func nanotime() int64 {
+	return time.Now().UnixNano()
+}