Add identifiers to variables.md

Author: chorman0773

src/variables.md

@@ -1,20 +1,27 @@
 # Variables
 
+r[variable]
+
+r[variable.intro]
 A _variable_ is a component of a stack frame, either a named function parameter,
 an anonymous [temporary](expressions.md#temporaries), or a named local
 variable.
 
+r[variable.local]
 A _local variable_ (or *stack-local* allocation) holds a value directly,
 allocated within the stack's memory. The value is a part of the stack frame.
 
+r[variable.local-mut]
 Local variables are immutable unless declared otherwise. For example:
 `let mut x = ...`.
 
+r[variable.param-mut]
 Function parameters are immutable unless declared with `mut`. The `mut` keyword
 applies only to the following parameter. For example: `|mut x, y|` and
 `fn f(mut x: Box<i32>, y: Box<i32>)` declare one mutable variable `x` and one
 immutable variable `y`.
 
+r[variable.init]
 Local variables are not initialized when allocated. Instead, the entire frame
 worth of local variables are allocated, on frame-entry, in an uninitialized
 state. Subsequent statements within a function may or may not initialize the