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Expand out varargs functions for seq / par

This idea came from @sweeboonlim and neatly addresses a couple of type
related problems we currently have with par and seq. The idea is to
expand out the varargs for `seq` and `par` into several overloaded
methods.

Benefits:

- Type safety for multi-arg `seq` and `par` (we did not have this
  before)
- For `seq` we actually have stronger type-safety than we had with any
  other approach: now we can assert that the result type of `seq`
  matches the last element in the sequence.
- For `par` we can assert that each task extends from a general type,
  making `par` neatly type-safe and covariant.

Downsides:

- Some method duplication (due to expanded varargs)
- Definite backward compatibility breakage: usage that used an array
  input to `seq` or `par` would now have to use `vaseq` and `vapar`
  which are the varargs, type-unsafe versions.

Version 1.0.0