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>

rust/flexbuffers/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+name = "flexbuffers"
+version = "0.1.0"
+authors = ["Casper Neo <cneo@google.com>"]
+edition = "2018"
+license = "Apache-2.0"
+
+[dependencies]
+serde = "1.0.103"
+serde_derive = "*"
+byteorder = "1.3.2"
+num_enum = "0.4.1"
+debug_stub_derive = "0.3.0"
+bitflags = "1.2.1"
+
+[dev-dependencies]
+bencher = "*"
+quickcheck = "*"
+quickcheck_derive = "*"
+rand = "*"