Epic: complete support for erasable Checked C syntax

[dtarditi]

We have the following work items to do:

• Create an optional header file: #496
• Create several examples of using the optional header file in code that compiles with and without a compiler that supports Checked C.: #497
• Create Wiki documentation explaining how to write code using the erasable code. issue #495
• Update the Checked C specification to describe the new syntax. #494

Issue #470 provides some feedback on the overall idea.