core/convert: Ensure Go -> CUE conversion respects struct field order

This applies both to conversion of Go values, and Go types.

To do this, we need to inject fake position information into the adt and
ast nodes that are created by the conversion. We need to make sure that
adt nodes have their ast source fields correctly populated, and we need
to ensure Vertexes have correct Structs. All of these are needed to
ensure that toposort has enough information to be able to find and use
position information.

Fixes #3554.

Signed-off-by: Matthew Sackman <[email protected]>
Change-Id: Ifdc2cf802e69699aa75490d3bb610470b20cb38a
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1207908
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: cuematthew

cue/testdata/benchmarks/issue2176.txtar

@@ -60,14 +60,14 @@ output: {
 }
 -- out/eval/stats --
 Leaks:  90
-Freed:  4079
-Reused: 4074
-Allocs: 95
+Freed:  684
+Reused: 680
+Allocs: 94
 Retain: 1359
 
-Unifications: 4161
-Conjuncts:    9489
-Disjuncts:    4804
+Unifications: 766
+Conjuncts:    3242
+Disjuncts:    1409
 -- out/evalalpha --
 (struct){
   #Datastream: (#struct){

cue/testdata/benchmarks/issue572.txtar

@@ -11,14 +11,14 @@ unified: {
 }
 -- out/eval/stats --
 Leaks:  0
-Freed:  4501
-Reused: 4497
-Allocs: 4
+Freed:  3001
+Reused: 2998
+Allocs: 3
 Retain: 0
 
-Unifications: 4501
-Conjuncts:    8998
-Disjuncts:    4501
+Unifications: 3001
+Conjuncts:    7498
+Disjuncts:    3001
 -- out/eval --
 (struct){
   _entries(:p): (#list){