fix(gnovm): ignore blank fields in struct equality and map keys

[omarsy]

Summary

Fixes Go 1.17 spec mismatches in Gno struct equality and map keys.

The Go spec says struct equality compares corresponding non-blank fields. Gno was also comparing fields named _, so values like:

struct{ _ int; X int }{1, 2} == struct{ _ int; X int }{3, 2}

incorrectly evaluated to false.

Gno also encoded blank struct fields into map keys, so a lookup such as:

m := map[struct{ _ int; X int }]string{{1, 2}: "a"}
m[struct{ _ int; X int }{3, 2}]

missed the existing entry even though the keys are equal under the Go spec.

This updates StructKind equality and struct map-key encoding to skip blank fields while preserving existing comparability checks. A companion negative test verifies that blank fields still count for comparability, so a struct containing _ []int remains non-comparable.

Spec reference:
https://go.googlesource.com/go/+/refs/tags/go1.17.5/doc/go_spec.html#Comparison_operators

Tests

go test ./gnovm/pkg/gnolang -run 'TestFiles/types/cmp_struct_[hi]' -count=1 -v
go test ./gnovm/pkg/gnolang -run 'TestFiles/types/cmp_struct' -count=1
go test ./gnovm/pkg/gnolang -run 'TestFiles/(map48|types/cmp_struct_h|types/cmp_struct_i)' -count=1 -v
go test ./gnovm/pkg/gnolang -run 'TestFiles/map4' -count=1
go test ./gnovm/pkg/gnolang -run 'TestComputeMapKey' -count=1
git diff --check

Note: a full go test ./gnovm/pkg/gnolang -count=1 was started before the map-key follow-up but did not complete after several minutes, so it was stopped.

AI Disclosure

This PR was prepared with AI assistance.

[Gno2D2]

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[davd-gzl]

It miss the test where all keys are blank: struct{ _, _ int }
This is a behavioral break for any persisted code using blank key. This should not be a problem