Env: provide new step2 shape, which steps to a value at the end of a shape

the semantics of Env.xyc for \step is very awkward when using, because \step directly
jumps to the end value of a segment. this means that Env.xzc([[0, 0, \step], [1, 1, \step]])
will have the value 1 between time 0 and 1.
using \step2 instead of \step, the value between 0 and 1 will be 0, and it will jump to 1
at time 1

Signed-off-by: Tim Blechmann <tim@klingt.org>