Report progress of an operation in the echo area.
REPORTER should be the result of a call to `make-progress-reporter'.
If REPORTER is a numerical progress reporter---i.e. if it was
made using non-nil MIN-VALUE and MAX-VALUE arguments to
`make-progress-reporter'---then VALUE should be a number between
MIN-VALUE and MAX-VALUE.
If REPORTER is a non-numerical reporter, VALUE should be nil.
This function is relatively inexpensive. If the change since
last update is too small or insufficient time has passed, it does
nothing.