[MLton] The evils of importing as 'extern'

[Stephen Weeks]

> It seems to me that there must have been some reson not to include
> "platform.h" in "c-chunk.h", which is where all the FFI declarations
> used to live.

The reason is that we want to keep the namespace pollution to a
minimum in generated C files, which include c-chunk.h.  Generated C
files include names that come from user code via _import declarations,
so including platform.h would greatly increase the chance of
conflicts.