[pallet-revive] Add balance_of syscyall for fetching foreign balances (#5675)

This adds an API method `balance_of`, corresponding to the
[BALANCE](https://www.evm.codes/#31?fork=cancun) EVM opcode.

In `Ext`, `balance` and `balance_of` are internally routed through the
same new `account_balance` method: `balance` is technically the same as
`balance_of` with the caller address. This avoids duplicating all the
tests and avoids a small inefficiency (in theory, `balance` directly
call `balance_of` however this introduces a round trip of converting the
target address to a H160 and back).

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Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>
Signed-off-by: xermicus <cyrill@parity.io>
Co-authored-by: command-bot <>