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/RustTest.sh

@@ -31,7 +31,16 @@ else
     exit 1
 fi
 
-cargo run $TARGET_FLAG --bin=alloc_check
+cargo run $TARGET_FLAG --bin=flatbuffers_alloc_check
+TEST_RESULT=$?
+if [[ $TEST_RESULT  == 0 ]]; then
+    echo "OK: Rust heap alloc test passed."
+else
+    echo "KO: Rust heap alloc test failed."
+    exit 1
+fi
+
+cargo run $TARGET_FLAG --bin=flexbuffers_alloc_check
 TEST_RESULT=$?
 if [[ $TEST_RESULT  == 0 ]]; then
     echo "OK: Rust heap alloc test passed."