Fixed the crash bug with RETX* into native functions

Most importantly, eel_v_receive() is only for EEL functions!
C function VM stack frames don't have limbo lists, so random
evil will happen.

Cleaned up the XRETURN handling code a bit while at it.

Added a comment explaining why we're handling return values
slightly differently for C functions. (Because I had to
figured it out all over again...)

Removed the temporary load() workaround in builtin.eel.

Closes #18.