Speed up the oil-native build

[andychu]

Getting it working is the main priority, but compilation times are blowing up. See metrics for release 0.8.1:

http://www.oilshell.org/release/0.8.1/benchmarks.wwz/ovm-build/

https://oilshell.zulipchat.com/#narrow/stream/121540-oil-discuss/topic/Release.200.2E8.2E1

It's especially slow under Clang.

I suspect what happened:

• new tiny headers included from every file: mylib.h -> common.h and -> gc_heap.h
  • we probably want to combine everything into one header. Is that possible?
• OBJ_HEADER() macro? Everything descents from Obj too.

Need to do some build profiling ...

(TODO: reorder build benchmaks too ...)

[andychu]

Huge post on compile times: https://randomascii.wordpress.com/2020/03/30/big-project-build-times-chromium/

We might not fall in that category, but the tools are interesting ...