Lifecycle supports 32-bit architectures

[staltz]

Context

I'm using Neon to write a Node.js native module that will be run in nodejs-mobile, on mobile architectures such as armeabi-v7a (32 bit), aarch64 and so forth.

Problem

With neon 0.10.1:

Cross-compilation for Android armv7 (32-bit) fails because AtomicU64 is not supported for that architecture. This is assuming that neon's napi-6 feature is enabled, because of

neon/crates/neon/src/sys/mod.rs

Lines 34 to 35 in fe4642e

	#[cfg(feature = "napi-6")]
	pub mod lifecycle;

and

neon/crates/neon/src/lifecycle.rs

Line 15 in fe4642e

atomic::{AtomicU64, Ordering},

If I use napi-5 feature, compilation succeeds.

Solution

This PR enables napi-6 to support 32-bit architectures, by using AtomicU32 instead. Arguably, we lose a lot more identifiers for InstanceId, but U32 gives us 4 billion identifiers, and maybe this is sufficient?

A follow-up to this PR would switch the AtomicU* type depending on the architecture.

[kjvalencik]

This seems reasonable. Since each of these represent a type of data that lives as long as the VM, it doesn't make sense to have this many.