feat: ObjectsMatch and JsonContentsMatch (unordered nested slices)

[sonnemusk]

Summary

Adds deep equality helpers that treat every slice/array as an unordered multiset (like ElementsMatch, but recursive into structs and maps). Addresses #806 and the JSON follow-up discussed on that issue.

type T struct{ Names []string }
assert.ObjectsMatch(t,
    T{Names: []string{"Joe", "Rick"}},
    T{Names: []string{"Rick", "Joe"}},
)

assert.JsonContentsMatch(t,
    `{"participants":["Joe","Rick"],"event":"Birthday party"}`,
    `{"event":"Birthday party","participants":["Rick","Joe"]}`,
)

Details

• Structs: exported fields only, field-by-field with the same rules
• Maps: key-wise, values compared with the same rules
• []byte compared as ordered bytes (not as a bag of bytes)
• Also available on assert.Assertions, require, and *f format variants

Fixes #806