Rework string expressions/casts

[jengelh]

Pull Request Type

• Build and Dependency changes

Description

Consolidate the somewhat ugly (const char *)expr.u8string().c_str() into a macro.

Checklist

• I have tested my changes locally and verified that they work as intended.
• I have reviewed the changes to ensure they do not introduce any unnecessary complexity or duplicate code.
• I understand that by submitting this pull request, I am agreeing to license my contributions under the project's license.

[winterheart]

Hi.
.u8string().c_str() casts is mostly temporary workaround until we completely change all APIs to std::filesystem::path and switch (someday, I hope) to C++20 where we can use std::format for string operations. After this these casts will be redundant.

[jengelh]

someday, I hope

Yeah and until that day, let's have nice things, shall we?

[jengelh]

At the very least, merge up to a2c9ec8.