Touch ups to Array (#475)

* Add return type to `asarray` (makes easy to use with other typed values)
* Include `asarray` in `pxd` for faster access within other Cython modules

Authors:
  - https://github.com/jakirkham
  - Mads R. B. Kristensen (https://github.com/madsbk)

Approvers:
  - Mads R. B. Kristensen (https://github.com/madsbk)

URL: #475

python/kvikio/kvikio/_lib/arr.pxd

@@ -28,6 +28,9 @@ cdef class Array:
     cpdef Py_ssize_t _nbytes(self)
 
 
+cpdef Array asarray(obj)
+
+
 cdef pair[uintptr_t, size_t] parse_buffer_argument(
     buf, size, bint accept_host_buffer
 ) except *

python/kvikio/kvikio/_lib/arr.pyx

@@ -289,9 +289,18 @@ cdef inline Py_ssize_t _nbytes(Py_ssize_t itemsize,
         nbytes *= shape_mv[i]
     return nbytes
 
-cpdef asarray(obj):
+
+cpdef Array asarray(obj):
+    """Coerce other objects to ``Array``. No-op for existing ``Array``s.
+
+    Args:
+        obj: Object exposing the Python buffer protocol or ``__cuda_array_interface__``
+
+    Returns:
+        Array: An instance of the ``Array`` class
+    """
     if isinstance(obj, Array):
-        return obj
+        return <Array>obj
     else:
         return Array(obj)