sealevel: support CPIs

Support cross-program invocations

`a22b9c85`	[sol_invoke_signed_c](https://bpf.wtf/sol-0x04-syscalls/#solinvokesignedc)
`d7449092`	[sol_invoke_signed_rust](https://bpf.wtf/sol-0x04-syscalls/#solinvokesignedrust)
