Polish, rename, and document stepwise functions

[yaxu]

WIP on #1207

[yaxu]

Can't work out why the expand tests are failing..

[yaxu]

I'll probably add backward compatibility on the names, even though they were 'experimental'..

[yaxu]

Probably not the final word on this but I might merge it for now?

If/when tidalcycles/Tidal#1111 is solved, I think things will become clearer..

[yaxu]

Renaming taper to shrink, and adding grow, changing behaviour of negative values to match, so shrink is progressive drop, grow is progressive take. Combine amount and times into single list param, with times defaulting to the tactus.

Making drop drop from the start for positive values and from the end for negative ones.