Add critical-section dependency and signal module

[gmmyung]

This adds Rust implementation of signal, raise, and abort to ANSI C signals: SIGTERM, SIGSEGV, SIGINT, SIGILL, SIGABRT, SIGFPE.

A few caveats:

• core::intrinsics::abort() is unstable, so the default handler panics.
• Technically, SIGSEGV, SIGILL, SIGFPE can be implemented using exception handlers, but I am not sure if this breaks things when used in other crates.

[thejpster]

I think this should be an optional dependency?

[thejpster]

OK, I had a look at this, and I had a look at the libc crate, and I think we can do this with atomic usize instead.

[thejpster]

How do you feel about #21 as an alternative?

[gmmyung]

Yes, #21 seems like a better implementation. Just one thing, there is a race condition while testing. So each test should be encapsulated in a critical section, or the test should be ran with—test-threads=1.

[gmmyung]

Also, I have excluded the signal feature from default features because it requires critical-section. Do you think it is okay to enable it by default now?

[thejpster]

I'd make it optional because it takes 64 bytes of RAM.

[thejpster]

I'll read up on tests with shared mutable state.

[gmmyung]

I'd make it optional because it takes 64 bytes of RAM.

I don't know if the rust linker strips unused static variables. I thought it didn't, but I'm not so sure.