Move planned features to a new section in the specification

[dtarditi]

The following features aren't implemented in the Checked C compiler. They are described in the Checked C specification as though they are implemented and work. This is confusing for developers. Move them to a "Planned extension features" section in the specification.

• Pointer arithmetic overflow checking.
• Relative alignment.
• Support for by-reference parameter passing of array_ptr values.

[dtarditi]

In-progress: see https://github.com/checkedc/checkedc/tree/checkedc-v1 branch.