[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

rust/flexbuffers/src/lib.rs

@@ -44,12 +44,15 @@ mod bitwidth;
 mod builder;
 mod flexbuffer_type;
 mod reader;
+mod buffer;
+
 pub use bitwidth::BitWidth;
 pub use builder::Error as SerializationError;
 pub use builder::{
     singleton, Builder, BuilderOptions, FlexbufferSerializer, MapBuilder, Pushable, VectorBuilder,
 };
 pub use flexbuffer_type::FlexBufferType;
+pub use buffer::Buffer;
 pub use reader::Error as ReaderError;
 pub use reader::{DeserializationError, MapReader, Reader, ReaderIterator, VectorReader};
 use serde::{Deserialize, Serialize};
@@ -64,17 +67,32 @@ pub fn to_vec<T: Serialize>(x: T) -> Result<Vec<u8>, SerializationError> {
     x.serialize(&mut s)?;
     Ok(s.take_buffer())
 }
+
 /// Deserialize a type from a flexbuffer.
 pub fn from_slice<'de, T: Deserialize<'de>>(buf: &'de [u8]) -> Result<T, DeserializationError> {
     let r = Reader::get_root(buf)?;
     T::deserialize(r)
 }
 
+/// Deserialize a type from a flexbuffer.
+pub fn from_buffer<'de, T: Deserialize<'de>, B: Buffer>(
+    buf: &'de B
+) -> Result<T, DeserializationError> {
+    let r = Reader::get_root(buf as &'de [u8])?;
+    T::deserialize(r)
+}
+
 /// This struct, when pushed will be serialized as a `FlexBufferType::Blob`.
 ///
 /// A `Blob` is a variable width `length` followed by that many bytes of data.
-#[derive(Debug, Copy, Clone, PartialEq, Eq)]
-pub struct Blob<'a>(pub &'a [u8]);
+#[derive(Debug, PartialEq, Eq)]
+pub struct Blob<B>(pub B);
+
+impl<B: Buffer> Clone for Blob<B> {
+    fn clone(&self) -> Self {
+        Blob(self.0.shallow_copy())
+    }
+}
 
 /// This struct, when pushed, will be serialized as a `FlexBufferType::IndirectUInt`.
 ///