Add testing and benchmarking steps with --no-default-features

[pedrodesu]

Fixes #605.

[pedrodesu]

The code at the previous point imported Allocator and Global incorrectly from alloc (instead of alloc::alloc), thus it failed. Additionally, we must force the allocator_api feature if we're not using the default allocator-api2 feature.

I also changed the extern of std, it's redundant since it's under the feature of std. I believe it's better to just invert the logic and only toggle #![no_std] if the std feature isn't present.

My PR reflects these changes.

One thing to bear in mind: Our current code forces the usage of an allocator API, whether nightly allocator_api or allocator-api2's. This means that if a user on stable rust disables allocator-api2, our library would never compile as-is (indeed, if you hypothetically ran Cargo test no default features under stable, surely the action would fail).
Nothing needs to be changed if that's deliberate, but if not, we should probably add more conditional code to fallback to the API of no custom allocators if we don't detect either nightly or allocator-api2. This, of course, has the price of more code.

[alejandro-vaz]

thanks, looks good.