Implement transaction invariant checks (RIPD-1425):

Add new functionality to enforce one or more sanity checks (invariants)
on transactions. Add tests for each new invariant check. Allow
for easily adding additional invariant checks in the future.

Also Resolves
-------------

  - RIPD-1426
  - RIPD-1427
  - RIPD-1428
  - RIPD-1429
  - RIPD-1430
  - RIPD-1431
  - RIPD-1432

Release Notes
-------------

Creates a new ammendment named "EnforceInvariants" which must be
enabled in order for these new checks to run on each transaction.

src/ripple/app/tx/impl/ApplyContext.cpp

@@ -19,10 +19,12 @@
 
 #include <BeastConfig.h>
 #include <ripple/app/tx/impl/ApplyContext.h>
+#include <ripple/app/tx/impl/InvariantCheck.h>
 #include <ripple/app/tx/impl/Transactor.h>
 #include <ripple/basics/Log.h>
 #include <ripple/json/to_string.h>
 #include <ripple/protocol/Indexes.h>
+#include <ripple/protocol/Feature.h>
 #include <cassert>
 
 namespace ripple {
@@ -69,4 +71,56 @@ ApplyContext::visit (std::function <void (
     view_->visit(base_, func);
 }
 
+template<std::size_t... Is>
+TER
+ApplyContext::checkInvariantsHelper(TER terResult, std::index_sequence<Is...>)
+{
+    if (view_->rules().enabled(featureEnforceInvariants))
+    {
+        auto checkers = getInvariantChecks();
+
+        // call each check's per-entry method
+        visit (
+            [&checkers](
+                uint256 const& index,
+                bool isDelete,
+                std::shared_ptr <SLE const> const& before,
+                std::shared_ptr <SLE const> const& after)
+            {
+                // Sean Parent for_each_argument trick
+                (void)std::array<int, sizeof...(Is)>{
+                    {((std::get<Is>(checkers).
+                            visitEntry(index, isDelete, before, after)), 0)...}
+                };
+            });
+
+        // Sean Parent for_each_argument trick
+        // (a fold expression with `&&` would be really nice here when we move
+        // to C++-17)
+        std::array<bool, sizeof...(Is)> finalizers {{
+            std::get<Is>(checkers).finalize(tx, terResult, journal)...}};
+
+        // call each check's finalizer to see that it passes
+        if (! std::all_of( finalizers.cbegin(), finalizers.cend(),
+                [](auto const& b) { return b; }))
+        {
+            terResult = (terResult == tecINVARIANT_FAILED) ?
+                tefINVARIANT_FAILED :
+                tecINVARIANT_FAILED ;
+            JLOG(journal.error()) <<
+                "Transaction has failed one or more invariants: " <<
+                to_string(tx.getJson (0));
+        }
+    }
+
+    return terResult;
+}
+
+TER
+ApplyContext::checkInvariants(TER terResult)
+{
+    return checkInvariantsHelper(
+        terResult, std::make_index_sequence<std::tuple_size<InvariantChecks>::value>{});
+}
+
 } // ripple