[SPEED] investigate options for improving compilation time in inim

[timotheecour]

• if there is not adverse consequence on error messages (eg if we never need to look at bufferSource), would it make sense to get the source from from stdin instead of dumping to a temp file bufferSource ? nim can read from stdin using -

• curious also whether compiler/nimeval could speed things up (calling compiler as a library instead of shelling out)

• just posted https://forum.nim-lang.org/t/4048 : "how to get detailed compilation timing statistics (time spent in each phase, nim=>C=>.o=>binary) ? " to see whether we can measure bottlenecks in compile time speed