Support native extensions

[mikedanese]

local nativeExtension = import src/jsonnet_regexp.h;

dlopens a libarary, calls jsonnet_regexp_registered_functions() (or some other well-known name), which returns a list of builtins to register and declare in the nativeExtension object.

[sparkprime]

That's very specific to the implementation language (C++).

But we could do is something like
std.builtin("foo")
and require the application code linking the jsonnet library to provide callbacks (just like we already allow import callbacks for doing I/O).

There is already an AST node for builtins, but it is only introduced during desugaring, it is not part of the actual syntax of the language. We could make it part of the actual syntax and allow adding more builtins as well.

[sparkprime]

Example here: https://gist.github.com/sparkprime/5b2ab0a1b72beceab2cf5ea524db228c

[mikedanese]

This is awesome!

[vitiral]

I think this is related: https://groups.google.com/forum/#!topic/jsonnet/OhWV2RKlUFE

I didn't see this documented at all in the standard library so I started that thread. Also, I still don't see any documentation about "std.native"