Get rid of push/commit/init syscalls

[playX18]

Proposal

On last call everyone agreed it is a good idea to get rid of syscalls like send_commit, send_push and others. Rationale for this is that these syscalls:

• Hardly used by anyone
• Hard to maintain: these syscalls need to preserve state of partially constructed messages between some actions e.g gr_wait and we need to maintain this invariant in the entirety of code.

What should users do?

If there's any users of these syscalls they should implement similar mechanism of constructing partial messages on their own using static storage e.g static mut in Rust.