Borrowed Accounts API Consolidation

[cali-jumptrading]

Match Agave borrowed account semantics where possible. These changes include the following:

• rename fd_borrowed_account.h to fd_txn_acct.h. This file will define a transaction account with borrowing checks (read/write exclusion)
• Add fd_borrowed_account_t, which is a wrapper around fd_txn_acct that includes the instruction context, the index of the instruction / transaction account... matching agave as close as possible. Link to agave: https://github.com/firedancer-io/agave/blob/943bd6bbafc85c729eb276dcc0564f5fbcca9814/sdk/src/transaction_context.rs#L703
• Move APIs in fd_account.h to either fd_borrowed_account.h or fd_exec_instr_ctx.h, depending on whether they act upon the borrowed account or upon the instruction context. This will more closely match agave as well, for example, this link shows the set_lamports function in the BorrowedAccount class: https://github.com/firedancer-io/agave/blob/943bd6bbafc85c729eb276dcc0564f5fbcca9814/sdk/src/transaction_context.rs#L773
• Remove FD_BORROWED_ACCOUNT_TRY_BORROW_IDX macro and add those borrow checks into the account borrowing APIs in fd_exec_instr_ctx.h and fd_exec_txn_ctx.h.