fix(lua): record `candidates_size` in `scanner_new_str`
fix(lua): record `candidates_size` in `scanner_new_str`

So that we can `munmap()` with the right size.

This is obviously fragile, as it depends on the one call site that uses
this function on using the same size (that's the Watchman scanner). I'm
ok with that, as there will probably never be any other call sites.

wincent/command-t,wincent/command-t,wincent/command-t