Add seq._(fn, args...) member function

[tcbrindle]

This allows adding custom operations as part of a pipeline using the "dot" syntax, for example

auto res = get_sequence()
            .filter(...)
            .map(...)
            ._(custom_op, arg1, arg2)
            .fold(...)

which will call custom_user_op(seq, arg1, arg2) with the adapted sequence as the first argument, followed by the provided args.

I previously had this in Flow as x.apply(fn, args...), but Barry's idea from his Rivers library to name it just x._(fn, args...) is much better.

[codecov[bot]]

Codecov Report

Base: 96.93% // Head: 96.93% // No change to project coverage :thumbsup:

Coverage data is based on head (9f24e50) compared to base (48dedb3). Patch has no changes to coverable lines.

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Additional details and impacted files

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