Rust Flexbuffers (#5669)

* Cargo clippy lints

* more lints

* more lints

* Restored a doc comment

* Comment on float eps-eq and adjusted casting

* Rust Flexbuffers

* more serde tests, removed some unsafe

* Redid serde to be map-like and Reader is Display

* Moved iter from Reader to VectorReader

* Serious quickcheck + bugs

* wvo api

* Made types smaller for a reasonable speedup

* redid reading in a way that's a bit faster.

Profiling shows the rust slowdown as building +10%, reading +20%

* src/bin are developer binaries in rust

* Root and Map width are not packed

* key null check is debug only + doc changes

* BuilderOptions

* Documentation

* Documentation

* Moved tests to rust_usage_test

* Moved rust flexbuffers samples to Flatbuffers/samples

* Fixed RustTest

* Fixed for Rust 1.37.0

* Upgraded to rust 1_40_0

* fixed a little-endian-only feature in a test

* 1.40.0

* fixed some benchmarks for bigendian

* Updated .bat file

* misspelling

* Gold Flexbuffer test.

* Serialize,Deserialize, std::error::Error for Errors.

* Undo rustfmt in integration_test.rs

* from_slice instead of from_vec

* Added comments to unsafe blocks

* expanded on comment

* bump

Co-authored-by: CasperN <cneo@google.com>

tests/rust_usage_test/Cargo.toml

@@ -5,23 +5,31 @@ authors = ["Robert Winslow <hello@rwinslow.com>", "FlatBuffers Maintainers"]
 
 [dependencies]
 flatbuffers = { path = "../../rust/flatbuffers" }
+flexbuffers = { path = "../../rust/flexbuffers" }
 
 [[bin]]
 name = "monster_example"
 path = "bin/monster_example.rs"
 
 [[bin]]
-name = "alloc_check"
-path = "bin/alloc_check.rs"
+name = "flatbuffers_alloc_check"
+path = "bin/flatbuffers_alloc_check.rs"
 
+[[bin]]
+name = "flexbuffers_alloc_check"
+path = "bin/flexbuffers_alloc_check.rs"
 
 [dev-dependencies]
 quickcheck = "0.6"
 # TODO(rw): look into moving to criterion.rs
 bencher = "0.1.5"
 static_assertions = "1.0.0"
+serde_derive = "*"
+serde = "*"
+rand = "*"
+quickcheck_derive = "*"
 
 [[bench]]
 # setup for bencher
-name = "flatbuffers_benchmarks"
+name = "benchmarks"
 harness = false