Compute arguments iterator immediately, instead of on first use

[BenWoodworth]

Currently in v0.1, a parameter's arguments wait to be computed until it's first used. Especially in the case of the lazy parameter {...}, unused parameters won't have their arguments calculated, saving on unnecessary computation.

However, this blocks a few desired features:

1. Avoiding re-calculation of independent parameter

See #3. Having a gap between a parameter's declaration and when its arguments are calculated means dependence between parameters can't be tracked, making it impossible to add an optimization for re-using an arguments iterable. Immediately calculating the arguments would enable this optimization

2. Parameters that are only used after the iteration completes

See #10. There is reason to calculate arguments even if the parameter isn't used by the time the iteration completes. So if arguments are going to be calculated anyway, then that might as well happen right away as that's most intuitive. That would break the current behavior, but if we need the argument, the options are break the current behavior and compute them at a different time, or keep the current behavior and slip the computation in at the end. Which... would be both unintuitive and needlessly complicated with no real benefit.

3. Iterating arguments upon declaration

See #12.