Issue Description:

We provide several operations which expose the guts of an Array as an UnsafePointer in a closure. This provides an opportunity for developers to mess up and scribble past the bounds of the Array. As a QoI feature, we should look into opportunistically installing a canary at the end of the Array in debug builds.

Basic idea:

• At the start of withUnsafeBufferPointer (before the closure is called), check if the array has some slack capacity.

• If it does, write some specific bit pattern to the extra space (just the last byte?).

• At the end of withUnsafeBufferPointer (after the closure has been called), check if the bit pattern is in tact. If not, assert that a buffer overflow occurred.

This obviously isn't a robust protection against buffer overflows, but maybe it will catch some programmer errors!

I'm happy to mentor anyone who wishes to work on this.