CLN: move groupby algos separate cython lib

- separate out groupby algorithms to separate lib
- release GIL on median
- release GIL on is_lexsorted / fix memory leak
- release GIL on nancorr

Author: Jeff Reback <[email protected]>

Closes #15775 from jreback/groupby and squashes the following commits:

4e2bfec [Jeff Reback] release GIL on median release GIL on is_lexsorted / fix memory leak release GIL on nancorr
ce28bb5 [Jeff Reback] CLN: separate out groupby algorithms to separate lib

Author: jreback

pandas/_libs/algos.pxd

@@ -0,0 +1,13 @@
+from util cimport numeric
+from numpy cimport float64_t, double_t
+
+cpdef numeric kth_smallest(numeric[:] a, Py_ssize_t k) nogil
+
+cdef inline Py_ssize_t swap(numeric *a, numeric *b) nogil:
+    cdef numeric t
+
+    # cython doesn't allow pointer dereference so use array syntax
+    t = a[0]
+    a[0] = b[0]
+    b[0] = t
+    return 0