Fix Yul block execution after control-flow checkpoints

[smoelius]

This bug was found by Codex, and the PR was produced by Codex.

This PR fixes Yul block execution so that a block stops evaluating subsequent
statements once a statement produces a non-normal control-flow state, such as
the state produced by break, continue, or leave.

Previously, exec continued executing the rest of a .Block after any
successful Except.ok result, even if the returned Yul state was a control-flow
state rather than a normal .Ok state. That meant a block like:

{
  break
  let x := 1
}

would continue into let x := 1 after break, instead of stopping at the break
state.

The block execution case now continues only when the intermediate state is
.Ok; otherwise it returns the checkpoint state unchanged.

Testing

Added EvmYul.Yul.InterpreterTests.exec_block_stops_after_break, which checks
that:

{
  break
  let x := 1
}

evaluates to .Checkpoint (.Break ...) without executing the later let.

Tested with:

lake build EvmYul.Yul.InterpreterTests