Handle const qualifier in values, matchers, mocks

[malsyned]

I haven't checked out what issues might exist in supporting const qualified types. Ones I can think of right now include:

• Consistent, useful boxing conventions for values (indirect once and store as pointer-to-const-pointer?)
• Accepting pointers to them as arguments to mocks
• Matching them against other const data (the_const_memory, is_the_const_memory)
• Matching non-const data against them (the_memory, matches_the_const_memory)
• Matching them against non-const data? Is this even useful? (the_const_memory, matches_the_memory)
• Providing generic equality matcher functions like is_the_whatever_matcher

[malsyned]

This turned out to be a lot easier than I expected. There is only one place in the code where a boxed value is written to after creation, in assign_arg_do_copy(). If all value objects are treated as const by default, and assign_arg_do_copy() removes the const qualifier, everything works fine.

As far as I can tell, removing the const qualifier from a pointer that actually points to non-const data, then writing to it, is a well-defined operation in C.