GitLab

Lots of individual things:

- removed the "Begin*" functions; no need to expose that kind of raw
  operation when the callback forms are zero-cost.

- renamed functions for consistent "Build" vs "Assemble".

- added "Assign*" functions for all scalar kinds, which reduces the
  usage of "AbsorbError" (but also, left AbsorbError in).

- renamed the ListEntry/MapEntry functions to also have "Assemble*"
  forms (still callback style).

- while also adding Assign{Map|List}Entry{Kind} functions (lets you
  get rid of another callback whenever the value is a scalar).

- added Assign{|MapEntry|ListEntry} functions, which shell out to
  fluent.Reflect for even more convenience (at the cost of performance).

- moved higher level functions like CopyRange to a separate file.

- new benchmark, for the terser form of working with scalars.
  (It's also evidently slightly faster, because fewer small function
  calls.  Slightly surprising considering how much inlining we might
  expect, but, huh.  Alright, surprise bonus; acceptable.)

- example function updated to use the terser form.

With these terseness improvements to handling of scalars, the overall
SLOC count for using the quip system is now exactly on par with fluent.

Varations on map key arguments (which could be PathSegment or even
Node, in addition to string) still aren't made available this.
Perhaps that's just okay.  If you're really up some sort of creek
where you need that, you can still just use the
MapAssembler.AssembleKey system directly, which can do everything.