keccak256: check return code and optimize stack push

[tcrypt25519]

Two outstanding items in the keccak256 builder op (crates/jet/src/builder/ops.rs):

• ops.rs:613 — The return code of the keccak256 runtime call is not checked. A failure would be silently swallowed.
• ops.rs:620 — After the call the result pointer is pushed via __call_stack_push_ptr. Since the data is already in place we could instead simply advance the stack pointer.