[rust] Genericize flexbuffer reader (#6450)

* feature/rust-tokio-bytes added feature name for tokio-bytes

* Added flexbuffer implementation, TODO: typecast to avoid recurse

* Converted codebase to utilize FlexBuffer implementation, need to resolve deserialization issues

* Added todo for lifetime issue, may use &'de [u8] for deserializer instead of current method

* Added proper &[u8] implementation

* Removed unused struct

* Added experimental fix to get_slice

* Added experimental fix to get_slice

* Avoided lifetime issues via ref structs, need to check if this hurts peformance

* Updated deserializer implementation to allow for borrowed data from Reader struct

* Fixed bug with str

* Removed unnecessary generic parameter

* Added unsafe to avoid lifetime complaints, current tests pass, need to review alternatives to unsafe

* Opinionated: Removed bytes crate as this implementation could be done in a separate crate

* Cleaned up flatbuffer

* Fixed sample / example

* Resolved PR feedback, need to resolve issues with tests

* Cleaned up FlexBuffer trait to be an auto impl

* Removed TODO

* Reverted Deserializer to only support &'de [u8]

* Cleaned up / renamed function for clarification

* Renamed FlexBuffer -> InternalBuffer for clarification on it's purpose

* Fixed issue with key bytes

* resolved issues with broken tests, confirming this is a breaking change

* Removed FIXME that's solved by splitting String and Key variants

* Implemented associated types approach

* Fixed backward slice logic

* Fixed MapReader compile error

* Added from_buffer for deserialization, removed  function since it's only needed for deserialization

* Removed dead code

* Cleaned up buffer, removed AsRef in favor of Deref

* Renamed Buffer::as_str -> Buffer::buffer_str

* Minor cleanup

* Updated documentation, need to fix tests

* Removed unnecessary &

* Removed unused lifetime

* removed unnecessary as_ref

* Minor optimization wrap-up

* resolved issue with Clone

* Added test to verify no deep-copy

* Added  for optimization

* Updated to use empty fn instead of default

* Updated comments / test name - plus the 0.3.0 version bump

* comment

tests/rust_usage_test/benches/flexbuffers_benchmarks.rs

@@ -224,7 +224,7 @@ fn serialize_monsters(b: &mut Bencher) {
     b.iter(go);
     b.bytes = n as u64;
 }
-fn validate_monster(r: MapReader) {
+fn validate_monster(r: MapReader<&[u8]>) {
     assert_eq!(r.idx("type").as_str(), "great orc");
     assert_eq!(r.idx("age").as_u8(), 100);
     assert_eq!(r.idx("name").as_str(), "Mr. Orc");