refactor: change chainid from struct to enum

[eitanm-starkware]

This change is 

[ShahakShama]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @eitanm-starkware and @MohammadNassar1)

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src/core.rs line 29 at r1 (raw file):

}

impl Serialize for ChainId {

You have a lot of code duplication here. Impl From for String and then use the Froms in all other functions

[ShahakShama]

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @MohammadNassar1)