Enable crate to be used with the stable compiler

[dpaoliello]

• Make the _in functions optional based on the allocator_api feature.
• Add the use_unstable_apis feature to disable other unstable API usage.
• By default, the above features will be enabled.
• Workaround the lack of a stable way to create TryReserveError using mem::transmute.
• Workaround the lack of support for #[cfg] on generic parameters using macros.
• Switched the CI build to use build.ps1.
• Added tests for building with different feature combinations enabled, including with no features enabled without RUSTC_BOOTSTRAP set.