StepDecay and OneCycle accept values that divide by zero or underflow

[CanReader]

Two gaps in src/optim/schedule.rs:

• StepDecay::new(base, 0, gamma) then panics with "attempt to divide by zero" on
  the first lr_at (schedule.rs:51). Nothing validates every in the
  constructor.
• OneCycle with warmup_fraction > 1.0 makes warmup > total, so
  self.total - warmup underflows usize (schedule.rs:140). Panic in debug,
  nonsense remaining in release, since the release profile has overflow checks
  off.

Every other builder in optim/ already asserts its range (Adadelta::rho,
RMSprop::alpha, Lookahead::new), these two just got missed.