Rollup merge of #61780 - SimonSapin:container-error, r=Amanieu

Finalize the error type for `try_reserve`

See tracking issue comments from https://github.com/rust-lang/rust/issues/48043#issuecomment-500828346.

It is now:

```rust
/// The error type for `try_reserve` methods.
#[derive(Clone, PartialEq, Eq, Debug)]
#[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
pub enum TryReserveError {
    /// Error due to the computed capacity exceeding the collection's maximum
    /// (usually `isize::MAX` bytes).
    CapacityOverflow,

    /// The memory allocator returned an error
    AllocError {
        /// The layout of allocation request that failed
        layout: Layout,

        #[doc(hidden)]
        #[unstable(feature = "container_error_extra", issue = "0", reason = "\
            Enable exposing the allocator’s custom error value \
            if an associated type is added in the future: \
            https://github.com/rust-lang/wg-allocators/issues/23")]
        non_exhaustive: (),
    },
}

#[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
impl From<LayoutErr> for TryReserveError {
    #[inline]
    fn from(_: LayoutErr) -> Self {
        TryReserveError::CapacityOverflow
    }
}
```

Changes:

* A `Layout` is included. Firefox wants to log the size of failed allocations. If this were not part of the return value of e.g. `HashMap::try_reserve`, users would only be able to estimate based on `HashMap::capacity` and assumptions about the allocation strategy of `HashMap`.

* There’s a dummy field that can stay unstable when `try_reserve` and the rest of this enum are stabilized. This forces non-exhaustive matching ~(https://github.com/rust-lang/rust/issues/44109 is not implemented yet for variants)~ and allows adding another field in the future if we want to expose custom error values from the allocator. See https://github.com/rust-lang/wg-allocators/issues/23.

  - If the `Alloc` trait is stabilized without an associated error type and with a zero-size `AllocErr` type, we can simply remove this dummy field.
  - If an associated type is added, we can add a default type parameter to `ContainerError` and a generic field to the `AllocError` variant.

* ~Moved from the `collections` module to the `alloc` module, and replaced `Collection` in the enum name with `Container`. The wold collection implies a multiplicity of items which is not relevant to this type. For example we may want to use this error type in a future `Box::try_new` method.~

  - Renamed to `TryReserveError`, after the methods that involve this type: https://github.com/rust-lang/rust/pull/61780#issuecomment-501392487

* Replaced `Err` with `Error` in the enum and variant names. There is more precedent for this in https://doc.rust-lang.org/std/error/trait.Error.html#implementors, `AllocErr` and `LayoutErr` are the odd ones.

* ~Dropped `Alloc` in the enum name. `ContainerAllocError` with a mouthful, and being in the `alloc` module already provides the same indication.~

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