random numbers aren't sampled at onset

[yaxu]

PR #1148 hasn't fixed the behaviour of rand:

ghci> queryArc (rand :: Pattern Double) (Arc 0 0)
[~0>0~|0.0]
ghci> queryArc (rand :: Pattern Double) (Arc 0 0.1)
[~0>⅒~|0.9219377599656582]
ghci> queryArc (rand :: Pattern Double) (Arc 0 0.3)
[~0>3/10~|5.677513778209686e-2]

Also note that the first value is 0. When random numbers are sampled at onset, this will cause problems for weighted functions, ref tidalcycles/strudel#1293