Consider removing uses of colon in Checked C syntax

[dtarditi]

We have introduced new keywords beginning with _ for the Checked C bounds annotations and where clauses, as part of creating erasable syntax. We no longer need the colon (:) to declare bounds declarations and where clauses from in declarations - there is not a potential for collisions. We should consider changing the Checked C syntax to no longer require the colon.

If we do this, we need to:

• update tests to remove the use of the ':' and to use the new keywords.
• update the syntax descriptions on the Wiki and in the spec to deprecate usage of the :.
• update the 3C tools and various real-world codebases. These include:
  • https://github.com/checkedc/checkedc-vsftpd
  • https://github.com/checkedc/checkedc-tiny-bignum-c
  • https://github.com/checkedc/checkedc-thttpd
  • https://github.com/checkedc/checkedc-icecast
  • https://github.com/checkedc/checkedc-libjpeg-tutorial