More info on NimMain procedure

[awr1]

approximately, `NimMain`:

  * pulls in command line arguments
  * links to all `{.dynlib.}` procedures
  * if needed, starts TLS emulation
  * if needed, start the GC
  * if needed, registers all nim-side procedures and globals with the HCR 
(hot-code reloader)
  * initalizes the nim-side callstack + exception handler
  * executes `system.nim`
  * executes user top-level runtime code