Our implementation has a more modern interface and only
supports the features that we care about.
It always outputs `\n` as newlines and does not output BOM.
The modern interface eliminates awkward `c_str()/data()` conversions.
This implementation preserves comments and the file order of sections
and keys. New keys are written in insertion order.
We now also support modifying and adding default comments,
which may be a useful thing to do for the especially tricky
ini options (this PR doesn't add any but adds the ability to do so).
Sadly, this increases the RG99 binary size by 24 KiB.
I'm guessing this is because the map implementation generates
quite a bit of code.
Note that while it might seem that using `std::string` for every key and
value would do a lot of allocations, most of these strings are
small and thus benefit from Small String Optimization (= no allocations).
We also add a direct string move-assignment (`operator=(std::string &&)`),
which results in better codegen.
As a consequence, we have to replace `= {}` assignments with `= StringOrView {}`
because `= {}` is now ambiguous.
1. Do not modify the map after loading. Instead, return string views
(guaranteed to be null-terminated) from look up functions and return
the key directly if not found.
2. Use an `unorded_map` instead of `map` where available (C++20).
Saves a bit of RAM (~50 KiB) and improves lookup performance.
Gleb Mazovetskiy