accounts/abi: simplify reflection logic (#21058)

* accounts/abi: simplified reflection logic

* accounts/abi: simplified reflection logic

* accounts/abi: removed unpack

* accounts/abi: removed comments

* accounts/abi: removed uneccessary complications

* accounts/abi: minor changes in error messages

* accounts/abi: removed unnused code

* accounts/abi: fixed indexed argument unpacking

* accounts/abi: removed superfluous test cases

This commit removes two test cases. The first one is trivially invalid as we have the same
test cases as passing in packing_test.go L375. The second one passes now,
because we don't need the mapArgNamesToStructFields in unpack_atomic anymore.
Checking for purely underscored arg names generally should not be something we do
as the abi/contract is generally out of the control of the user.

* accounts/abi: removed comments, debug println

* accounts/abi: added commented out code

* accounts/abi: addressed comments

* accounts/abi: remove unnecessary dst.CanSet check

* accounts/abi: added dst.CanSet checks

accounts/abi/event_test.go

@@ -312,14 +312,14 @@ func TestEventTupleUnpack(t *testing.T) {
 		&[]interface{}{common.Address{}, new(big.Int)},
 		&[]interface{}{},
 		jsonEventPledge,
-		"abi: insufficient number of elements in the list/array for unpack, want 3, got 2",
+		"abi: insufficient number of arguments for unpack, want 3, got 2",
 		"Can not unpack Pledge event into too short slice",
 	}, {
 		pledgeData1,
 		new(map[string]interface{}),
 		&[]interface{}{},
 		jsonEventPledge,
-		"abi: cannot unmarshal tuple into map[string]interface {}",
+		"abi:[2] cannot unmarshal tuple in to map[string]interface {}",
 		"Can not unpack Pledge event into map",
 	}, {
 		mixedCaseData1,