[meta] Wishlist for smallvec 2.0

[Matt Brubeck]

These are changes that weren't ready in time for 1.0 (#73).

Requires newer version of Rust than current minimum (1.36):

• Use const generics to change SmallVec<[T; N]> to SmallVec<T, N>. Remove the Array trait. (#240, Rust 1.51).
• Use the union representation by default and remove the optional union feature added in #94 (Rust 1.49).
• Make SmallVec::new a const fn (#263).

Depends on unstable Rust features:

• Support custom allocators (#55, rust-lang/rust#32838).
• Use specialization to optimize extend, insert_many, and from for slices of Copy types. Deprecate and remove extend_from_slice, insert_from_slice, and from_slice (rust-lang/rust#31844).
• Implement the Error trait without requiring std (#355, rust-lang/rust#103765).

Breaking API changes:

• Fix variance of SmallVec<[T; N]> over T (#146)?
• Remove the deprecated ExtendFromSlice trait.
• Remove unused &self parameter from inline_size method.
• Remove the non-standard insert_many method (#255)?
• Deprecate the write feature and add a std feature instead (#355).