Move libfsm/start.c to Rust

[federicomenaquintero]

This depends on #343.

First we replace the bitfields in a couple of structs with stdbool.h, so we can have the same ABI as Rust's bool type.

Then we make parallel structs in Rust for struct fsm_state and struct fsm. We'll keep these in sync with the C structs while the internals get rustified.

Then we port fsm_clearstart, fsm_getstart, fsm_setstart to Rust, and remove start.c.

I probably totally botched the Makefiles to add -lpthread -ldl, needed by Rust libraries, but I need some help from someone who knows pmake :)

[federicomenaquintero]

The last commits I've pushed (fsm_addstate() onwards) are a little refactoring of some of the C code. My goal here is to isolate the use of fsm->statealloc to fsm.c, which should hopefully make it easier to turn the states array into a Rust Vec.

Those commits are useful outside of a Rust port. Please tell me if you'd rather have me submit a separate pull request for them, or if you'd prefer to cherry-pick them or whatever.