Solana: make anchor account.fetch() work

[seanyoung]

Anchor can decode the fields in the account data of anchor rust programs. The metadata contains a list of fields. Make this work with Solidity.

A complication is that Anchor uses borsh encoding for the account data. Solidity can't use borsh encoding for this, as we want to update dynamic length items in place (e.g. strings) without re-encoding everything.

[LucasSte]

This is the same thing as in #541 and #1192

[seanyoung]

We could merge this work with walking the account storage directly without copying. This work may change the storage layout.

[seanyoung]

This could be related #933