Add move semantics to MessageBuilder, FlatBufferBuilder, SliceAllocator, and vector_downward (#4893)

Unit tests
Update flatbuffers + gRPC build instructions
Update CMakeLists.txt with cmake variables for grpc and protobuf install paths
Update tests for travis build

include/flatbuffers/grpc.h

@@ -88,6 +88,22 @@ class SliceAllocator : public Allocator {
   SliceAllocator(const SliceAllocator &other) = delete;
   SliceAllocator &operator=(const SliceAllocator &other) = delete;
 
+  SliceAllocator(SliceAllocator &&other)
+    : slice_(other.slice_) {
+    other.slice_ = grpc_empty_slice();
+  }
+
+  SliceAllocator &operator=(SliceAllocator &&other) {
+    slice_ = other.slice_;
+    other.slice_ = grpc_empty_slice();
+    return *this;
+  }
+
+  void swap(SliceAllocator &other) {
+    using std::swap;
+    swap(slice_, other.slice_);
+  }
+
   virtual ~SliceAllocator() { grpc_slice_unref(slice_); }
 
   virtual uint8_t *allocate(size_t size) override {
@@ -151,6 +167,29 @@ class MessageBuilder : private detail::SliceAllocatorMember,
   MessageBuilder(const MessageBuilder &other) = delete;
   MessageBuilder &operator=(const MessageBuilder &other) = delete;
 
+  MessageBuilder(MessageBuilder &&other)
+    : FlatBufferBuilder(1024, &slice_allocator_, false) {
+    // Default construct and swap idiom.
+    Swap(other);
+  }
+
+  MessageBuilder &operator=(MessageBuilder &&other) {
+    // Move construct a temporary and swap
+    MessageBuilder temp(std::move(other));
+    Swap(temp);
+    return *this;
+  }
+
+  void Swap(MessageBuilder &other) {
+    slice_allocator_.swap(other.slice_allocator_);
+    FlatBufferBuilder::Swap(other);
+    // After swapping the FlatBufferBuilder, we swap back the allocator, which restores
+    // the original allocator back in place. This is necessary because MessageBuilder's
+    // allocator is its own member (SliceAllocatorMember). The allocator passed to
+    // FlatBufferBuilder::vector_downward must point to this member.
+    buf_.swap_allocator(other.buf_);
+  }
+
   ~MessageBuilder() {}
 
   // GetMessage extracts the subslice of the buffer corresponding to the