Feature request: interop API

[jakkn]

As a consumer of this library and writing code in languages other than Nim, I'd like to read and write files through a simple API.

Nim's bidirectional interfacing promises simple interfacing with C like targets, but an API must be defined and a library build must be shipped. The README already documents an API but the types, such as GffRoot, are complex and challenging to get right when crossing languages. A simpler API with basic data types such as cstring would make interoperability easier.

Example API:

proc readJsonFromGff*(gffFilename: cstring): cstring {.cdecl, exportc, dynlib.}
proc writeJsonToGff*(json: cstring, gffFilename: cstring) {.cdecl, exportc, dynlib.}

Example use from C#:

[DllImport("libnwn_gff.so", CallingConvention = CallingConvention.Cdecl)]
private static extern IntPtr readJsonFromGff(
  [MarshalAs(UnmanagedType.LPUTF8Str)] string gffFilename
);

[DllImport("libnwn_gff.so", CallingConvention = CallingConvention.Cdecl)]
private static extern void writeJsonToGff(
  [MarshalAs(UnmanagedType.LPUTF8Str)] string json,
  [MarshalAs(UnmanagedType.LPUTF8Str)] string gffFilename
);

It seems useful to me that the API be defined and shipped with this project, but I may be missing some considerations. I also understand the maintenance implied by defining APIs, so please let me know how you feel about this. If you agree, I've already worked out the implementation and can PR the changes.

[niv]

Some lower-level shims sound like a good idea for me. If you need them, I'm happy to review a PR.