EVM: debugger use Lwt_fmt instead of Lwt_io for output

Context

The debugger uses Lwt_io.printf which doesn't flush when encountering %!. (cf manual) . This can be an issue with the benchmark script, which greps the output concurrently, so if a line isn't complete, info will be missed.

This MR replaces Lwt_io.print* by Lwt_fmt.printf which does take into account %!

Manually testing the MR

Only change is how the output looks during execution. One way to see a change is to run the debugger slowly, for example by doing some profiling, and seeing the lines being output one by one.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR