Adds smart pivot selection to key-value sorting

[sterrettm2]

Adds smart pivot selection to key-value sorting, improving performance in some cases like when one value is extremely common. This gives around a 10x performance improvement when half the data is 0.

[r-devulap]

uh oh, i remember seeing this failure in #134 as well. Any idea why this happens?

/opt/rh/devtoolset-10/root/usr/include/c++/10/bits/stl_vector.h:1045: std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::operator[](std::vector<_Tp, _Alloc>::size_type) [with _Tp = short unsigned int; _Alloc = std::allocator<short unsigned int>; std::vector<_Tp, _Alloc>::reference = short unsigned int&; std::vector<_Tp, _Alloc>::size_type = unsigned int]: Assertion '__builtin_expect(__n < this->size(), true)' failed.

[r-devulap]

LGTM. Thanks @sterrettm2 !