Use Buffers in Auto-Completion

[flatcap]

Here's the layout of functions in Auto-Completion:

Source: graphviz, svg

On the left, buf_get_field() and its 66 callers take a struct Buffer parameter.
Everything to the right needs converting.

Spoiler

Start by changing the EnterWindowData to use both a Buffer and char* (they'd point to the same memory).
This would allow you to convert each of the Editor's functions as a separate commit.
e.g. <op_editor_complete> / op_editor_complete().

The complete_*() functions take EnterWindowData, so they will only need tweaking as you convert the actual completion functions, on the right.

[roccoblues]

This has been done in 02a97ee and can be closed right?

[flatcap]

Yes.
The next stage is to factor out all the functions on the right.
That's been adopted by Amudtogal in #3955 Neomutt completion API