function fillBy( x, fcn, thisArg ) {
	map( [ x, x ], fcn, thisArg );
}
Fills an input ndarray according to a callback function.

@param {ndarrayLike} x - ndarray-like object
@param {string} x.dtype - data type
@param {Collection} x.data - data buffer
@param {NonNegativeIntegerArray} x.shape - dimensions
@param {IntegerArray} x.strides - stride lengths
@param {NonNegativeInteger} x.offset - index offset
@param {string} x.order - specifies whether `x` is row-major (C-style) or column-major (Fortran-style)
@param {Function} fcn - callback function
@param {*} [thisArg] - callback function execution context
@returns {void}

@example
var Float64Array = require( '@stdlib/array/float64' );

function fcn() {
    return 10.0;
}

// Create a data buffer:
var xbuf = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );

// Define the shape of the input array:
var shape = [ 3, 1, 2 ];

// Define the array strides:
var sx = [ 2, 2, 1 ];

// Define the index offset:
var ox = 0;

// Create the input ndarray-like object:
var x = {
    'dtype': 'float64',
    'data': xbuf,
    'shape': shape,
    'strides': sx,
    'offset': ox,
    'order': 'row-major'
};

fillBy( x, fcn );

console.log( x.data );
// => <Float64Array>[ 10.0, 10.0, 10.0, 10.0, 10.0, 10.0 ]