(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'.
(calc-do-quick-calc): Use `frame-width' instead of `screen-width'.
(calcAlg-edit): Use `minibuffer-contents' instead of `buffer-string'.
(calcAlg-enter): Likewise.
(calcAlg-enter): Use `minibuffer-prompt-end' instead of `point-min'.
