Add links for arena and interning. (#1868)

Co-authored-by: Yuki Okushi <jtitor@2k36.org>

src/memory.md

@@ -4,7 +4,10 @@ Rustc tries to be pretty careful how it manages memory. The compiler allocates
 _a lot_ of data structures throughout compilation, and if we are not careful,
 it will take a lot of time and space to do so.
 
-One of the main way the compiler manages this is using arenas and interning.
+One of the main way the compiler manages this is using [arena]s and [interning].
+
+[arena]: https://en.wikipedia.org/wiki/Region-based_memory_management
+[interning]: https://en.wikipedia.org/wiki/String_interning
 
 ## Arenas and  Interning
 

src/overview.md

@@ -246,12 +246,15 @@ for different purposes:
   optimizations on it.
 
 One other thing to note is that many values in the compiler are _interned_.
-This is a performance and memory optimization in which we allocate the values
-in a special allocator called an _arena_. Then, we pass around references to
-the values allocated in the arena. This allows us to make sure that identical
-values (e.g. types in your program) are only allocated once and can be compared
-cheaply by comparing pointers. Many of the intermediate representations are
-interned.
+This is a performance and memory optimization in which we allocate the values in
+a special allocator called an
+_[arena]_. Then, we pass
+around references to the values allocated in the arena. This allows us to make
+sure that identical values (e.g. types in your program) are only allocated once
+and can be compared cheaply by comparing pointers. Many of the intermediate
+representations are interned.
+
+[arena]: https://en.wikipedia.org/wiki/Region-based_memory_management
 
 ### Queries