Remove in-place optimizations of patch and merge

[mfelsche]

Pull request

Description

This PR removes the patch and merge in-place optimizations, as they can lead to memory unsafety.

a script like:

# initialize state
let state = match state of
  case null => {}
  default state
end;
let state = merge state of event end;
emit state

will happily mutate state and insert values from event, which will no longer be valid after the event is gone, and so allow a view into freed up or reallocated memory via string keys or values.
We try to get back to those optimizations in a safer way.

Related

• Related Issues: fixes transmute in _in_place functions #1032

Checklist

• The RFC, if required, has been submitted and approved
• Any user-facing impact of the changes is reflected in docs.tremor.rs
• The code is tested
• Use of unsafe code is reasoned about in a comment
• Update CHANGELOG.md appropriately, recording any changes, bug fixes, or other observable changes in behavior
• The performance impact of the change is measured (see below)

Performance

[Licenser]

Nice catch! 🚀

[coveralls]

Coverage increased (+0.02%) to 85.244% when pulling 70f7e11 on remove_in_place into bffc2a5 on main.

[darach]

LGTM 👍

[mfelsche]

For performance we see a slight increase for patch and merge with the optimization removed, which is honestly surprising.