This PR makes all the StoredAccountMeta fields pub(crate)
and provides getter functions to access its member fields.
This PR is a preparation step for abstracting out StoredAccountMeta.
#### Problem
As we start supporting new storage formats, those account-meta
structs will be shared by different storage formats and thus need
a new home.
#### Summary of Changes
This PR creates meta.rs under account_storage and moves all the account-meta
structs out from append_vec.rs.
Yueh-Hsuan Chiang
Jeff Washington (jwash)
apfitzge
Michael Vines
sakridge