lang: export accounts module, re-export its types in the prelude

[paul-schaaf]

changes:

• export accounts module. Make prelude re-export structs from accounts module instead of redefining them. From now on, account types are no longer available under the root but have to be imported from the prelude or the accounts module
• remove deprecated accounts from prelude (I thought this made sense so new users dont use it but it's not required for this PR)

partially fixes #1206