Do not use exprt::move_to_operands as it is marked as deprecated

[tautschnig]

In particular, the use in std_code.h triggers a series of warnings every time
this file is included.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 141b5e5).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/89882763

[smowton]

In #3067 the solution arrived at was to have a rvalue-ref version of add. Rather oddly this was implemented by creating rvalue-ref overloads of copy_to_operands (https://github.com/diffblue/cbmc/blob/develop/src/util/expr.h#L126), so we end up writing some_expr.copy_to_operands(std::move(other_expr)).

I'd suggest simply renaming copy_to_operands(exprt &&) as move_to_operands(exprt &&) or add_to_operands(exprt &&) and use that function to implement this PR.

[smowton]

PR here: #3254

[tautschnig]

I have removed the changes to codet from this commit/PR, I will do them separately based on the work that @smowton has done in the meantime. Hopefully this is now no longer controversial.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: ec5e0be).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/89942582