Add a skeleton Data Representation chapter

Author: Alan Jeffrey

reference/src/SUMMARY.md

@@ -13,5 +13,6 @@
   - [Uninhabited types like `!` and exhaustiveness](./active_discussion/uninhabited_types.md)
   - [Unions](./active_discussion/unions.md)
   - [Uninitialized memory](./active_discussion/uninitialized_memory.md)
+- [Data representation](./representation.md)
 - [Optimizations](./optimizations.md)
   - [Optimizing immutable memory](./optimizations/immutable_memory.md) 

reference/src/representation.md

@@ -0,0 +1,24 @@
+# Data structure representation
+
+## Introduction
+
+## Representation of scalars
+
+https://github.com/rust-rfcs/unsafe-code-guidelines/issues/9
+
+## Representation of structs and tuples
+
+https://github.com/rust-rfcs/unsafe-code-guidelines/issues/11
+https://github.com/rust-rfcs/unsafe-code-guidelines/issues/12
+
+## Representation of enums 
+
+https://github.com/rust-rfcs/unsafe-code-guidelines/issues/10 
+
+## Representation of unions
+
+https://github.com/rust-rfcs/unsafe-code-guidelines/issues/13
+
+## Representation of function pointers
+
+https://github.com/rust-rfcs/unsafe-code-guidelines/issues/13