New sample collector core (Pf2c)

[osyoyu]

This branch experiments with a new sample collector implemented in C.
The goal is to reimplement the sample collector, which is currently written in Rust. The committed version still lacks many features from the original implementation.

Why reimplement?

• The original implementation's quality is not necessarily high.
  • It overuses mutexes/rwlocks.
    • And it does not employ proper AtomicBool synchronization where it should.
  • Deadlocks occur under certain conditions. Which is a big problem.
  • SignalScheduler's installation procedure is super messy.
• Rust issues
  • There is no rustc in most environments, especially in containers.
    • rb-sys does automatically download rustc and cargo on gem install, however that consumes a lot of time, and compilation is still slow.
  • Tons of glue code is required bridging Rust and C interfaces (especially ruby.h macros).

Of course I can improve these problems one by one, but given that C is the lingua franca of the Ruby community, I'd guessed that trying C wouldn't be that bad. And it wasn't (at least for now).

How to use

As pf2c still lacks many features, it is gated behind conditional compilation. Users must run gem install with the environment variable PF2_PF2C=1. This will disable the original Pf2, and in turn expose the Pf2c constant to Ruby.