Rust: Add non_exhaustive attribute to all unions and enums #136
Description
https://doc.rust-lang.org/reference/attributes/type_system.html#the-non_exhaustive-attribute
cc @graydon @stellar/contract-committers
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https://doc.rust-lang.org/reference/attributes/type_system.html#the-non_exhaustive-attribute
cc @graydon @stellar/contract-committers