Returns the number of values included in this extent, as a usize, if the
elements of the extent and the quantity of values between them,
inclusively, can all be represented exactly as a usize. Various
conditions may make this impossible: if the extent uses a numeric type
larger than a usize, for example, or even if it includes the entire
range of possible usize values (the quantity of which are a number one
greater than the largest representable usize). This function also
returns None for any extents involving negatve values of signed types
(some such cases could still be represented as usize differences from
lo-to-hi, but it is fussy to get the cases all right and this is not the
main use-case for this library).