feat: new allocator features

[alejandro-vaz]

this PR revamps the allocator features

feature hierarchy

1. no features = custom allocator implemented
2. allocator-api = requires nightly, uses the API
3. allocator-api2 = pulls the API from crates.io

the allocator-api2 implies allocator-api, to make sure no weird case of both together when they should be disjoint exists

exported types

each of the three modes internally exports three types: Global, Vec and Box

which are then used in the rest of the API

that means that all conversions only happen to and from those types, not others

this is relevant to #649, though allocator-handling is a bit fucked up in the base case, so I'm not sure if it'd be easy to handle it there. and it unblocks #644

closes #655

closes #631

note: there is a regression in from_elem because I couldn't use the vec macro anymore without doing macro wizardry, so it now uses iterators again

[alejandro-vaz]

well I think it makes more sense to invert the dependency dependencies of the two allocator ones and have the allocator-api2 as the default

[alejandro-vaz]

ah no, can't do that because the nightly one then will randomly download allocator-api2 which is pointless

[alejandro-vaz]

I've disabled one fuzzing operation because it magically failed to compile

we have to remake fuzzing anyway so I'm not worried, see #657

[alejandro-vaz]

I will fix the fuzzing stuff once #658 lands

[bolshoytoster]

note: there is a regression in from_elem because I couldn't use the vec macro anymore without doing macro wizardry, so it now uses iterators again

I've been able to get it to work by just exporting the macro from the allocator/ modules. Although I have to add #[rustfmt::skip] to stop it from merging

pub use alloc::{
    vec::Vec,
    vec
};

into

pub use alloc::{
    vec::{
        self,
        Vec
    }
};

[alejandro-vaz]

yeah that's exactly what I didn't want to do

do we have any other alternative??

maybe

#[rustfmt::skip]
pub use alloc::vec;

[bolshoytoster]

Yeah I think that should work too.

[bolshoytoster]

Does this PR mean you can only From/Into the Vec exposed by allocator?

Given that the default would be allocator-api2::vec::Vec, would it be nice to at least make sure SmallVec<T, N, Global> from/into alloc::vec::Vec<T, Global> are supported?

[alejandro-vaz]

that's a good point

maybe by default we should disable all features and that's it honestly

very few people use custom allocators yet anyway

it'd also make CI simpler

[pedrodesu]

I've disabled one fuzzing operation because it magically failed to compile

we have to remake fuzzing anyway so I'm not worried, see #657

What did it complain about?

[alejandro-vaz]

actually I think it'll work now