SatInstance::to_dimacs_path slow (unbuffered)

[suomela]

Some benchmarks on macOS (using a 23MB uncompressed CNF file):

This took ≈ 30 seconds:

instance.to_dimacs_path(filename)?;

This also took ≈ 30 seconds:

let mut f = File::create(filename)?;
instance.to_dimacs(&mut f)?;

But this was only ≈ 0.2 seconds:

let f = File::create(filename)?;
let mut f = BufWriter::new(f);
instance.to_dimacs(&mut f)?;

It looks like to_dimacs_path should wrap things in BufWriter; now it is apparently doing just File::create + to_dimacs, which can be very slow.

[suomela]

PS. While at it, I noticed that open_compressed_uncompressed_write uses a bit confusingly-named variable raw_reader.