Calling Bend from C

[o1lo01ol1o]

Is your feature request related to a problem? Please describe.

I'd like to be able to call bend programs from the c-runtime so that I can use bend as a part of existing ecosystems.

Describe the solution you'd like

I'd like to write a bend program along with a header file that describes the given implementation, compile the bend program to c, then call the bend program from, eg, Unreal Engine.

Describe alternatives you've considered

Additional context

For cases where the bend program has accumulated state (ie, it's a scan or a fold in the host program), it'd be good to be able to leave the runtime alive so that it can be called at the subsequent step without rebuilding the computation.

[developedby]

Although not currently directly supported by us, it should be reasonably simple to call a bend program compiled to C externally.

Almost duplicate (but for C) of #627