Replaced clear-on-drop with 'std::ptr::write_volatile'.

[DrPeterVanNostrand]

• Removes the need to wrap SecretKeys in ClearOnDrop<Box<...>>.
• Allows for stack allocated SecretKeys.
• Zeros out the memory occupied by each SecretKey on drop.
• Ensures that the clearing of SecretKeys is not elided.
• Works on stable.

Closes Issues #89 and #90

[afck]

Perfect, that looks much cleaner and safer than having to remember using ClearOnDrop everywhere! 👍

[c0gent]

This looks good and zeros properly in my tests. It may be better in the future, however, to use a more thorough memory security library such as memsec/seckey. They will use a special allocator (like sodium) and do a few other things to make sure secure memory isn't swapped to disk, etc.

For now though this is a definite usability improvement over ClearOnDrop. Please merge or give me the ok to do so :)