[MRG+1] Sparse output KNN

- [x] ~~Implement Cython CSR rowise mode function in sparsefuncs_fast~~
- [x] ~~Implement Cython CSR rowise weighted_mode function in sparsefuncs_fast~~
- [x] ~~Backport scipy sparse advanced indexing~~


This PR originally started out with Cython based improvements, but a middle ground has been chosen for simplicity of implementation.
This logic should probably be moved to `LabelEncoding` at some point; it currently does not handle multioutput, nor sparse (but the latter had only been used for binary targets until now). The sparse implementation is not explicitly tested, and some of its conditions are only being tested because of the random number generation happening to produce entirely dense and non-dense columns.