External DLL support in Mojo

[sune-gaulsh]

It would be really awesome if Mojo supported calling functions from external dlls via external function pointers and passing pointers to preallocated memory. I am thinking C/C++/Fortran etc.

[lattner]

This should already work, but it is super low level. This is how we talk to the CPython interpreter for example.

[abduld]

@sune-gaulsh . Take a look at the external_call function (https://docs.modular.com/mojo/MojoStdlib/Intrinsics.html#external_call)