fix #442: use reflectwalk to walk through circuit structures without building a Schema

[gbotrel]

Closes #442 #433 #390.

Description

Introduces schema.Walk which behaves similarly to schema.Parse except it doesn't build a Schema, just visit the leaves.

Known issues:

• we still need schema.Parse to handle json witnesses (play.gnark.io), and it won't handle all circuit structures well
• schema.Walk doesn't like it if the leaf we are looking for is a reflect.Ptr. Only happens in... witness where we want to identify zero values in json will probably revisit when it's warranted.

Benchmark

Old (parse full schema) vs new (walk):

BenchmarkLargeSchema/walk-10                        4074            277284 ns/op           15456 B/op       4000 allocs/op
BenchmarkLargeSchema/parse-10                          1        2127391458 ns/op        3490130176 B/op 66707400 allocs/op
BenchmarkArrayOfSliceOfStructSchema/walk-10                    1        6265857250 ns/op        802062272 B/op  100257706 allocs/op
BenchmarkArrayOfSliceOfStructSchema/parse-10                   1        13797889083 ns/op       21742293688 B/op        351920063 allocs/op

[ivokub]

Seems to be OK.

I understand the change introduces a new way for walking the circuit struct to call some handler on the leafs (for initialising the variables). We do not otherwise change the behaviour (e.g. for compile we still walk several times to first get counts of variables and then build variables separately)? I think it is OK as walking for counting is quite fast.

There is now ParseDeprecated and LeafHandlerDeprecated. I didn't understand fully why they are still necessary, are they for marshalling/unmarshalling? Do we plan to fully deprecate or rewrite?

I do like that the visibility handling became simpler. Seems that it covers all the cases. At some point I fixed some problem when name was set but visibility not and then the parser assumed Secret visibility instead of inhereting, but it seems this is also now covered.

[gbotrel]

Yes behavior is unchanged, we may have some perf issues with very large complex circuits (with multi dimension slices and structs) at compile time, but nothing urgent to fix for now.

ParseDeprecated I will try to kill it and reuse a walker pattern to ensure we use the same exact code (reflection part). The only part where we use this method (probably better named as schema.New) is to marshal/unmarshall json witness. So for us, in play.gnark.io . --> not critical if slow for large circuits.