Port FlatBuffers to Rust (#4898)

This is a port of FlatBuffers to Rust. It provides code generation and a
runtime library derived from the C++ implementation. It utilizes the
Rust type system to provide safe and fast traversal of FlatBuffers data.

There are 188 tests, including many fuzz tests of roundtrips for various
serialization scenarios. Initial benchmarks indicate that the canonical
example payload can be written in ~700ns, and traversed in ~100ns.

Rustaceans may be interested in the Follow, Push, and SafeSliceAccess
traits. These traits lift traversals, reads, writes, and slice accesses
into the type system, providing abstraction with no runtime penalty.

rust/flatbuffers/src/lib.rs

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2018 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+mod builder;
+mod endian_scalar;
+mod follow;
+mod primitives;
+mod push;
+mod table;
+mod vector;
+mod vtable;
+mod vtable_writer;
+
+pub use builder::FlatBufferBuilder;
+pub use endian_scalar::{EndianScalar, emplace_scalar, read_scalar, read_scalar_at, byte_swap_f32, byte_swap_f64};
+pub use follow::{Follow, FollowStart};
+pub use primitives::*;
+pub use push::Push;
+pub use table::{Table, buffer_has_identifier, get_root, get_size_prefixed_root};
+pub use vector::{SafeSliceAccess, Vector, follow_cast_ref};
+pub use vtable::field_index_to_field_offset;
+
+// TODO(rw): Unify `create_vector` and `create_vector_direct` by using
+//           `Into<Vector<...>>`.
+// TODO(rw): Split fill ops in builder into fill_small, fill_big like in C++.