Add a section on assertions to the style guide.

corresponding `*_free` function. For example, list objects returned from

`list_new` should be freed with `list_free` and no other freeing routine.

### Asserts

Use `assert` liberally throughout the code to enforce invariants. Assertions

should not have any side-effects and should be used to detect programming logic

errors.

At minimum, every function should assert expectations on its arguments. The

following example demonstrates the kinds of assertions one should make on

function arguments.

```

  size_t open_and_read_file(const char *filename, void *target_buffer, size_t max_bytes) {

    assert(filename != NULL);

    assert(filename[0] != '\0');

    assert(target_buffer != NULL);

    assert(max_bytes > 0);

    // function implementation begins here

  }

```

## Header files

In general, every source file (`.c` or `.cpp`) in a `src/` directory should

have a corresponding header (`.h`) in the `include/` directory.