Deprecate `tic`,`tac`, `toc` timers

[mmcky]

Once the new context manager approach is added (#782) to QuantEcon.py we should look at deprecating the old timer functionality.

This would include

Step 1: enable DeprecationWarning in the current version of QuantEcon.py notifying of removal for version 0.9.*
Step 2: remove the functions when 0.9.* is released.