Add a slot_map::partition(Pred) member function

[Quuxplusone]

Just like the standard std::partition algorithm, this member function
reorders the slot_map's elements in such a way that all elements for which
the predicate pred returns true precede the elements for which pred
returns false. The relative order of the elements is not preserved.

The function returns an iterator to the first element of the second group
(which may be end(), in the case that all elements belong to the first group).

Addresses part of #131, although I'm not sure whether this really addresses
whatever use case @p-groarke is thinking of.

[p-groarke]

TBH, I'm not sure this is need. std::partition has worked great for my use cases so far.

[Quuxplusone]

TBH, I'm not sure this is need. std::partition has worked great for my use cases so far.

That would be great, but off the top of my head, would std::partition do the wrong thing because it would break the association between elements and keys? I mean https://wandbox.org/permlink/NdHnBhZgooDn0raP

stdext::slot_map<int> sm;
auto k1 = sm.insert(1);
auto k2 = sm.insert(2);
assert(sm.at(k1) == 1);
assert(sm.at(k2) == 2);
std::partition(sm.begin(), sm.end(), [](int x) { return x == 2; });
assert(sm.at(k1) == 2);
assert(sm.at(k2) == 1);

[p-groarke]

Oh yes your right, that's why we needed this. Sorry, been a long time :)

[p-groarke]

Comment and complexity would be nice to conform to the rest of the header.

[p-groarke]

Comments and complexity would be welcome.

[p-groarke]

@Quuxplusone Please take a look at #147
I discuss some issues I see in slot_map which would affect this PR. Cheers