Tests involving Bioconductor objects

[DarwinAwardWinner]

Currently all tests are using primitive vectors and/or lists. Since the goal is for the parallel functions to work on anything implementing c, [, and [[, we should add tests that work on Bioconductor vector-like classes, such as IRanges, SimpleList, GRangesList, and XStringSet, etc.

Issues to be addressed:

• How can we use a package in a test without making BiocParallel depend on that package?
• We should benchmark to see what kind of speedup is achieved with various backends. For a big complex object like GRanges, there might be a lot of overhead from transferring data between workers, as well as the splitting and joining operations. This wouldn't be part of the normal tests though.

[DarwinAwardWinner]

@ttriche here's the test issue. Feel free to add any self-contained test code that illustrates a use case.

[DarwinAwardWinner]

Here's an example test: 54cec90