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.

docs/source/FlatBuffers.md

@@ -4,7 +4,7 @@ FlatBuffers    {#flatbuffers_index}
 # Overview {#flatbuffers_overview}
 
 [FlatBuffers](@ref flatbuffers_overview) is an efficient cross platform
-serialization library for C++, C#, C, Go, Java, JavaScript, Lobster, Lua, TypeScript, PHP, and Python.
+serialization library for C++, C#, C, Go, Java, JavaScript, Lobster, Lua, TypeScript, PHP, Python, and Rust.
 It was originally created at Google for game development and other
 performance-critical applications.
 
@@ -144,6 +144,8 @@ sections provide a more in-depth usage guide.
     own programs.
 -   How to [use the generated Lobster code](@ref flatbuffers_guide_use_lobster) in your
     own programs.
+-   How to [use the generated Rust code](@ref flatbuffers_guide_use_rust) in your
+    own programs.
 -   [Support matrix](@ref flatbuffers_support) for platforms/languages/features.
 -   Some [benchmarks](@ref flatbuffers_benchmarks) showing the advantage of
     using FlatBuffers.