Improve performance when writing lp and reading sol files

[mbiggio]

We improve the performance of the solution file parser in the case of cplex, by making use of the more performant quick-xml crate and buffering the reads through BufReader. quick-xml indeed requires the reader to be buffered, probably to avoid making too many read syscalls.

Buffering is also applied to writing the lp file. Manual benchmarks are showing significant improvements over non-buffered read/writes for problem sizes of several thousands of variables

[mbiggio]

Lemme know if it might be worth adding some specific benchmark, maybe with some non-trivial .sol file to parse.

Anyway, just to provide some numbers, on my machine (Intel i5 processor, 10 cores, 32 GB RAM), solving a problem with:

• 184047 variables
• 306801 constraints

takes:

• ~13s without this commit
• ~8s with this commit

so a considerable amount of time was spent in IO, and this IO time is probably going to be proportional to the problem size

[lovasoa]

Merged; thank you!

[mbiggio]

Merged; thank you!

You're welcome!