IDEA: Use Unix-style chmod values for const and invariant

[Russell Lewis]

Ok, I was pondering, trying to figure out some sort of syntax which 
would allow us to communicate "const" and "invariant" in a way that's 
easy to learn & remember.  Maybe we could learn from UNIX file 
permissions, where you can see both the owner's permissions to the file, 
and "other's" permissions.  In that case, we have these comparisons:

const:		rw-r--r--
invariant:	r--r--r--

Which leads me to this concept for alternative keywords:

const:		const_wr
invariant:	const_rr

Sure, the first time that a C++ programmer sees this, he won't know 
exactly what it means, but it is (IMHO) easy to learn & remember.