Add a native compile target for demos

[paulcadman]

Add an option to compile a Juvix program to a native executable:

$ cd examples/milestone/HelloWorld
$ juvix compile -t native HelloWorld.juvix
$ ./HelloWorld
hello world!

This option is useful for demos because it does not depend on the Wasmer / Wasi environment.

[jonaprieto]

Tested and discussed in person.