Move require to end of file.

Move require to end of file.
(calc-info-filename, calc-autoload-directory, calc-extensions-loaded):
Remove unnecessary variable.
(calc-do-dispatch, calc-mode, calc-check-defines, calc-create-buffer)
(calc, calc-keypad, full-calc-keypad, calc-do, calc-normalize)
(calc-binary-op, calc-unary-op, calcDigit-start, calcDigit-key)
(math-normalize, math-add, math-mul, math-div,
(math-format-stack-value, math-stack-value-offset, math-format-value)
(math-format-flat-expr, math-format-number, math-format-bignum)
(math-read-number, calc-grab-region, calc-grab-rectangle)
(calc-grab-sum-down, calc-grab-sum-across, calc-embedded, defmath)
(calc-check-defines): Replace calc-need-macros by appropriate require.