Add WithContext: context and structured concurrency support

[destel]

Adds WithContext, giving a pipeline structured concurrency semantics similar to errgroup: rill derives the context, a sink cancels it as soon as its outcome is known (typically the first error), and the sink then waits for the pipeline to finish before returning.

ctx, scope := rill.WithContext(ctx)

// Source and other pipeline stages go here

users := rill.Map(ids, 10, func(id int) (*User, error) {
    return getUser(ctx, id) // context-aware
})

err := rill.ForEach(users, 5, func(u *User) error {
    return process(ctx, u)
}, scope)

// Nothing is running anymore

Without the option nothing changes: a sink still returns as soon as its outcome is known and drains the rest in the background.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.87%. Comparing base (d20890e) to head (f549696).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #113   +/-   ##
=======================================
  Coverage   99.87%   99.87%           
=======================================
  Files          17       16    -1     
  Lines         806      808    +2     
=======================================
+ Hits          805      807    +2     
  Misses          1        1           

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