Remove `PREXPR()` usage (as much as we can)

[lionel-]

It's essential for tidyeval. One way forward might to add an expr + env accessor to base R. That would return a list instead of a quosure (the latter would be harder to get in base R) but we could build quosures from this.

These should be fixable:

• SEXP tree walker. The memtools package still needs to traverse promises to find memory leaks but this package does not need to be on CRAN. So we can define the promise paths out based on a compilation flag. By default this would be commented out. Would fix vctrs.