Call return values

[lrettig]

To keep things simple, the call syscall was implemented without support for a return value (#5). We probably want to add additional syscalls with signatures like:

pub fn set_return_value(buf: *const u8, length: u64);
pub fn get_return_value(buf: *mut u8, length: u64);

EVM achieves this using the RETURN and REVERT opcodes. Solana has syscalls that are very similar to these. Here are some helpful resources:

• https://docs.solanalabs.com/proposals/return-data
• https://book.anchor-lang.com/anchor_in_depth/CPIs.html#returning-values-from-handler-functions
• https://docs.soliditylang.org/en/v0.8.26/units-and-global-variables.html#members-of-address-types