boolean <-> std_logic conversion functions

[ralight]

I find these functions very useful. Let's hope other people provide you with lots of other examples!

[martinjthompson]

Thanks for those - before I merge them, could you please add some testcases at the bottom (I know they're trivial, but it does flag up the fact that a ieee.std_logic_1164 clause hasn't been added in your pull request :)

Also, I assume you are happy for me to add you the list of "authors waiving copyright" in the header?

Thanks!