Divide items and expressions into sections

- Links in other parts of the reference were updated
- The following sections were moved inside others:
  * grouped expressions into operator-expr.md
  * while let into loop-expr.md
  * if let into if-expr.md
  * tuple indexing into tuple-expr.md

Author: brauliobz

src/SUMMARY.md

@@ -23,12 +23,42 @@
 
 - [Items and attributes](items-and-attributes.md)
     - [Items](items.md)
+        - [Modules](items/modules.md)
+        - [Extern crates](items/extern-crates.md)
+        - [Use declarations](items/use-declarations.md)
+        - [Functions](items/functions.md)
+        - [Type aliases](items/type-aliases.md)
+        - [Structs](items/structs.md)
+        - [Enumerations](items/enumerations.md)
+        - [Unions](items/unions.md)
+        - [Constant items](items/constant-items.md)
+        - [Static items](items/static-items.md)
+        - [Traits](items/traits.md)
+        - [Implementations](items/implementations.md)
+        - [External blocks](items/external-blocks.md)
     - [Visibility and Privacy](visibility-and-privacy.md)
     - [Attributes](attributes.md)
 
 - [Statements and expressions](statements-and-expressions.md)
     - [Statements](statements.md)
     - [Expressions](expressions.md)
+        - [Literal expressions](expressions/literal-expr.md)
+        - [Path expressions](expressions/path-expr.md)
+        - [Block expressions](expressions/block-expr.md)
+        - [Operator expressions](expressions/operator-expr.md)
+        - [Array and index expressions](expressions/array-expr.md)
+        - [Tuple and index expressions](expressions/tuple-expr.md)
+        - [Struct expressions](expressions/struct-expr.md)
+        - [Enum variant expressions](expressions/enum-variant-expr.md)
+        - [Call expressions](expressions/call-expr.md)
+        - [Method call expressions](expressions/method-call-expr.md)
+        - [Field access expressions](expressions/field-expr.md)
+        - [Closure expressions](expressions/closure-expr.md)
+        - [Loop expressions](expressions/loop-expr.md)
+        - [Range expressions](expressions/range-expr.md)
+        - [If and if let expressions](expressions/if-expr.md)
+        - [Match expressions](expressions/match-expr.md)
+        - [Return expressions](expressions/return-expr.md)
 
 - [Type system](type-system.md)
     - [Types](types.md)

src/attributes.md

@@ -137,8 +137,8 @@ On an `extern` block, the following attributes are interpreted:
 - `link` - indicate that a native library should be linked to for the
   declarations in this block to be linked correctly. `link` supports an optional
   `kind` key with three possible values: `dylib`, `static`, and `framework`. See
-  [external blocks](items.html#external-blocks) for more about external blocks. Two
-  examples: `#[link(name = "readline")]` and
+  [external blocks](items/external-blocks.html) for more about external blocks.
+  Two examples: `#[link(name = "readline")]` and 
   `#[link(name = "CoreFoundation", kind = "framework")]`.
 - `linked_from` - indicates what native library this block of FFI items is
   coming from. This attribute is of the form `#[linked_from = "foo"]` where