coding guidelines: prefer using instead of typedef

[kroening]

Functionally equivalent, but more modern. This is also what Google's C++
Style Guide says.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[tautschnig]

I am ok with this as long as we introduce it gradually rather than doing some rewrite of the codebase.

[chrisr-diffblue]

Happy with the update to the coding standard, and also echo Michaels comment that hopefully this won't encourage someone to do a giant re-rewrite PR :-)
Question: Should we update any clang-format/lint configs to match?

[tautschnig]

cpplint has a CheckNamespaceOrUsing rule, but it doesn't seem to be triggering much (we already have instances of using in the code base).

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: f2e8918).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/90286010