Speed up code generation in split_generated_files mode

[Millione]

Previously @missingdays supported split_generated_files mode to solve #454, but users reported that the generation speed was 3-4 times slower than the previous mode, and the total number of lines of generated code increased by about 20%.

Such a slowdown for large projects is indeed unacceptable, we need to investigate to find out where the slowdown is and how to fix it.

[Millione]

@missingdays Hi, as you suggested, we can tracking the issue here.

FYI: There's no detailed time analysis yet, but my guess is that it's mainly slow in creating, writing and formatting files, too many operations like this in split_generated_files mode

[missingdays]

@Millione Hi. Should we start with the measurements? With the generation taking several seconds, a simple

    let start = Instant::now();
    generate();
    let duration = start.elapsed();

should be precise enough.
It could measure the total execution time, as well as the time spent on each operation deemed expensive (e.g. file writing or formating). Then the measurements could be put into some sort of log.

[Millione]

@missingdays Totally agree, you can implement it on your branch and when you're ready I'll run it on the internal repository to get all the data we need. Then we can work on optimising it.