Refactor usages of std::ostringstream

[Baltoli]

A common pattern in the backend's code is to do the following:

std::ostringstream out;
pattern->print(out);
do_something(out.str());

In #911, we introduced a helper function ast_to_string that wraps this common pattern up; this PR follows up that change by mechanically refactoring the remainder of the codebase to use the helper. After the change, the above code would be:

do_something(ast_to_string(*pattern));

Each commit in the PR (bar the first) applies the same conceptual refactoring to a single file; no actual behavioural changes other than in the first commit are implemented, and so this should be an easy PR to review.

In future work, I'd like to further clean up this code and make better use of fmt across the codebase, but for now this is a useful self-contained change to make.

[gtrepta]

LGTM