program: require onramp to exist for deposit and withdraw

[2501babe]

i should have thought of this before but maybe splitting up #580 and #601 made me neglectful. we use onramp lamports for pool value now so we should just hard require it to exist. i considered working around its possible nonexistence but this is pointless. people will want their mev rewards, just make the account

with onramp, the pool_net_asset_value() change is no different, its just a bit more defensive. without onramp, the current version of pool_net_asset_value() is wrong, because it subtracts Rent::get().minimum_balance(0) from the pool value

requiring the onramp saves us from having to contemplate stupid edge cases like "what if it doesnt exist and has rent exemption for 0 but not enough for 200, is that pool value? what if it has enough for 200, is that pool value? what about more than that. what if it doesnt exist but they create it after. what if it has some lamports, but not enough for 0, but they add more after." enough

also i have some kind of brain disease that causes me to abbreviate "net asset value" as "nev" for some reason, so i also fix that

[joncinque]

Looks great! I always thought you meant it to be "net effective value" 😅