More flatness info through annotations

- Make functions be flat if their annotations are flat (as opposed to just any
  or blank).

- Pass information about the flatness of annotations into the annotation
  constructors. This replaces all the information about cheapness and
  "hasRefinement" that was there before. Now annotations simply know if they
  are flat or if they are not, and it matches the static interpretation (by
  definition, since makeFlatPredAnn is the only source of non-flatness, and it
  is only called when statically the annotation is determined to be flat).

- Harmonize the "fast" paths, which are actually semantically important. If the
  callee flatness optimization is applied, we need to ensure that nothing can
  result in a Continuation, so we cannot call safeCall in the annotation check.
  So these fast paths now exactly match the static flatness information they
  were instantiated with.

- Note that we're keeping dynamic flatness information around in the
  annotations for now, because the representations themselves can benefit from
  this information, even if we statically don't know. That is, it's safe for an
  annotation to use its fast path, even if at a particular static use we cannot
  guarantee it (say because we didn't keep the information across modules)

- As an additional optimization, make it so 

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# On branch ann-flatness
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#	modified:   src/arr/compiler/anf-loop-compiler.arr
#	modified:   src/arr/compiler/flatness.arr
#	modified:   src/js/base/runtime.js
#	modified:   src/scripts/show-compilation.arr
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#	forward-refinement.arr
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