Rust full reflection (#8102)

* #Rust Create a crate for reflection

* #Rust Add a crate for reflection tests and helper to access schema

* #Rust Get root table of a buffer and access field with schema

* #Rust Add 'Struct' struct and corresponding getter

* #Rust Add functions of getting any table/struct field value as integer/float/string

* #Rust Add setters for scalar fields

* #Rust Add setter for string fields

* #Rust Add getter for Table/Vector fields

* #Rust Add buffer verification

* Add a 'SafeBuffer' struct which provides safe methods for reflection

It verifies buffer against schema during construction and provides all the unsafe getters in lib.rs in a safe way

---------

Co-authored-by: Derek Bailey <derekbailey@google.com>

rust/reflection/src/struct.rs

@@ -0,0 +1,61 @@
+/*
+ * Copyright 2018 Google Inc. All rights reserved.
+ *
+ * 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
+ *
+ *     http://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 flatbuffers::Follow;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct Struct<'a> {
+    buf: &'a [u8],
+    loc: usize,
+}
+
+impl<'a> Struct<'a> {
+    #[inline]
+    pub fn buf(&self) -> &'a [u8] {
+        self.buf
+    }
+
+    #[inline]
+    pub fn loc(&self) -> usize {
+        self.loc
+    }
+
+    /// # Safety
+    ///
+    /// [buf] must contain a valid struct at [loc]
+    #[inline]
+    pub unsafe fn new(buf: &'a [u8], loc: usize) -> Self {
+        Struct { buf, loc }
+    }
+
+    /// Retrieves the value at the provided [byte_loc]. No field in [Struct] is optional.
+    ///
+    /// # Safety
+    ///
+    /// The value of the corresponding slot must have type T
+    #[inline]
+    pub unsafe fn get<T: Follow<'a> + 'a>(&self, byte_loc: usize) -> T::Inner {
+        <T>::follow(self.buf, self.loc + byte_loc)
+    }
+}
+
+impl<'a> Follow<'a> for Struct<'a> {
+    type Inner = Struct<'a>;
+    #[inline]
+    unsafe fn follow(buf: &'a [u8], loc: usize) -> Self::Inner {
+        Struct { buf, loc }
+    }
+}