(toplevel): Require calc-macs.

(toplevel): Require calc-macs.
(calc-minibuffer-size): New.
(calcDigit-nondigit): Use it instead of `buffer-size'.
(calcDigit-backspace): Likewise.
(calcDigit-nondigit): Use `minibuffer-contents' instead of `buffer-string'.
(calc-minibuffer-contains): Use `minibuffer-prompt-end' instead of `point-min'.
(calcDigit-key): Use `calc-minibuffer-contains' instead of a `save-excursion'.