[meta] Wishlist for smallvec 2.0

[mbrubeck]

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. (RFC: rewrite based on min_const_generics for 2.0 #240, Rust 1.51).
• Use the union representation by default and remove the optional union feature added in Use a union to reduce the size of SmallVec [v2] #94 (Rust 1.49).
• Make SmallVec::new a const fn (Make SmallVec::new const #263).

Depends on unstable Rust features:

• Support custom allocators (Support parametric allocators #55, Allocator traits and std::heap 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 (Tracking issue for specialization (RFC 1210) rust-lang/rust#31844).

Breaking API changes:

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