cue: deprecate Merge
cue: deprecate Merge

Fixes #703

Change-Id: Ieb8f7fd04c1846cebc451b1ddc287f0b241ec6af
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8461
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>

cuelang/cue