Add a section to the style guide on variable declarations.

Note that the function explicitly includes `void` in its parameter list to

indicate to the compiler that it takes no arguments.

### Variable declarations

Variables should be declared one per line as close to initialization as possible.

In nearly all cases, variables should be declared and initialized on the same line.

Variable declarations should not include extra whitespace to line up fields. For

example, the following style is preferred:

```

  int my_long_variable_name = 0;

  int x = 5;

```

whereas this code is not acceptable:

```

  int my_long_variable_name = 0;

  int                     x = 5;

```

As a result of the above rule to declare and initialize variables together,

`for` loops should declare and initialize their iterator variable in the

initializer statement:

```

  for (int i = 0; i < 10; ++i) {

    // use i

  }

```

### Contiguous memory structs

Use C99 flexible arrays as the last member of a struct if the array needs

to be allocated in contiguous memory with its containing struct.