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/src/reader/iter.rs

@@ -0,0 +1,63 @@
+// Copyright 2019 Google LLC
+//
+// 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
+//
+//     https://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.
+
+use super::{Reader, VectorReader};
+use std::iter::{DoubleEndedIterator, ExactSizeIterator, FusedIterator, Iterator};
+
+/// Iterates over a flexbuffer vector, typed vector, or map. Yields [Readers](struct.Reader.html).
+///
+/// If any error occurs, the Reader is defaulted to a Null flexbuffer Reader.
+pub struct ReaderIterator<'de> {
+    pub(super) reader: VectorReader<'de>,
+    pub(super) front: usize,
+    end: usize,
+}
+impl<'de> ReaderIterator<'de> {
+    pub(super) fn new(reader: VectorReader<'de>) -> Self {
+        let end = reader.len();
+        ReaderIterator {
+            reader,
+            front: 0,
+            end,
+        }
+    }
+}
+impl<'de> Iterator for ReaderIterator<'de> {
+    type Item = Reader<'de>;
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.front < self.end {
+            let r = self.reader.idx(self.front);
+            self.front += 1;
+            Some(r)
+        } else {
+            None
+        }
+    }
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let remaining = self.end - self.front;
+        (remaining, Some(remaining))
+    }
+}
+impl<'de> DoubleEndedIterator for ReaderIterator<'de> {
+    fn next_back(&mut self) -> Option<Self::Item> {
+        if self.front < self.end {
+            self.end -= 1;
+            Some(self.reader.idx(self.end))
+        } else {
+            None
+        }
+    }
+}
+impl<'de> ExactSizeIterator for ReaderIterator<'de> {}
+impl<'de> FusedIterator for ReaderIterator<'de> {}