fix: Make fields in tuple structs public

[cryptopapi997]

Currently when defining a tuple struct in an external program such as struct T(pub u32), declare_program! will transform this into struct T(u32). This effectively means initializing T in the program importing it isn't possible. This also differs from how normal structs work where each field is automatically made public. This PR changes this for tuple structs and makes each field of the tuple public too.

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