Add await_many and related functions

[joakin]

For running many tasks in parallel, it is complicated to get a solid solution, for example:

pub fn parallel(make_tasks: List(fn() -> a), timeout: Int) -> List(a) {
  make_tasks
  |> list.map(fn(make_task) { task.async(make_task) })
  |> list.map(fn(t) { task.await(t, timeout) })
}

Works but the timeouts will be run in sequence which is not what you really want.

It has been mentioned in discord a few times too:

• https://discord.com/channels/768594524158427167/768594524158427170/1287851279925907509
• https://discord.com/channels/768594524158427167/768594524158427170/1079034165024014388

[lpil]

Some of these functions have been added in v0.13