Add a way to import code from other files

[MilkeeyCat]

import "path" is used for importing local files
import "@srs_json/decoder" is used for importing files from external
packages

To let the compiler know that there's external package, use
--package /home/user/path/to/package, the path should point to a folder where
there will be 2 files: package.o and path.

package.o is compiled package as object file which will be just linked with
the user's program.
path file should contain path to source code of the package so it's possible
to look for a file there.