Reentrant variants of IGRAPH_FINALLY_PV()

Implement, for the R glue only, reentrant variants of the IGRAPH_FINALLY* API:

- Declare a stack local to the function with a new `IGRAPH_LOCAL_FINALLY_STACK` macro
- IGRAPH_LOCAL_* variants of the API that operate on the local stack
- Rewrite all functions in `interface_extra.c` to use the new API
- Adapt Stimulus to emit the new API, run code generation