Force keys to str instances

[asvetlov]

Right now a key type is dark corner: it could be str, istr or something other.
For sake of further optimizations it should be str always.

Passing istr as key to any miltidict API is still allowed and encouraged: it saves execution time by getting rid of s.title() conversion.

But istr type is ambiguous by it nature: we can define equality for str and istr by comparing their canonical representations but ordering is cumbersome.
Ordering by canonical reprs leads to very tricky errors, the same for original reprs.
Also CPython is very optimized for str but not classes derived from it.

I believe the change doesn't touch our users but aiohttp might get speedup on implementing the issue.