Create compose function for AccountSharedData

[LucasSte]

Problem

We want to remove fn create from WritableAccount, because the new account format for SVM has only references and does not allow one to create it from owned values, like the function in the trait.

I tracked all monorepo usages, and found that create is only used for AccountSharedData. There is a single usage for struct Account, but all its fields are public, so we can create it directly. See LucasSte/agave@224a93e

Solution

Create fn compose in impl AccountSharedData to replace fn create.

[brooksprumo]

I don't think we need a new constructor.

We should be able to use AccountSharedData::create_from_existing_shared_data() in all the places.

Or for the tests, they could create an Account first, then convert to AccountSharedData if needed, via Into/From.