Placement refactor (#6034)
## Summary
This PR is a refactoring of placement.rs. The code got more consistent,
some comments were updated and some dead code was removed or replaced
with debug assertions. It also contains a bugfix for the placement of
end-of-branch comments with nested bodies inside try statements that
occurred when refactoring the nested body loop.
## Test Plan
The existing test cases don't change. I added a couple of cases that i
think should be tested but weren't, and a regression test for the bugfix
