Using `furrr::future_map` instead of `purrr::map`

[talegari]

@markjrieke Great effort with the package!

Changing every purrr::map to furrr::future_map (in general, every purrr function with equivalent furrr function) will help in:

1. Using parallel processing with user preferred backend via future (runtime user setting).
2. Will fall back to serial (same as purrr::map) when parallel backend is not set.

[markjrieke]

This is a great suggestion, thank you!