Re-add tuna_assert_static

Reverts and builds atop commit 7a92025ab88082b83387eb6ed8beab9a58a04dfa.
Cleaner memcpy solution for solving #4.

The prior two-memcpy-calls solution was a bit ugly and more operations
than necessary if the compiler didn't optimize what was happening.  This
improves the clarity, especially in the comments, and it may simplify
things for the optimizer.