COSMIT refactor sklearn.utils.arrayfuncs

* use fused types and memoryviews
* turn boundschecking off
* make cholesky_delete.c a header file; it's included, not compiled (+ static inline declaration for safety)
* stylistic stuff (4 spaces for indent, not 3)

I don't usually do refactoring on Cython code because it bloats the repo, but new contributors are taking over our bad habits (#1412).