Make alloc optional

[burdges]

It might be handy if we'd some way to make alloc optional. One solution would be if users conditionally reexported smallvec and arrayvec.

#[cfg(feature = "alloc")]
pub use smallvec::SmallVec as TInyVec;

#[cfg(not(feature = "alloc"))]
pub use arrayvec::ArrayVec as TinyVec;

It's possible the smallvec and arrayvec crates should be brought into closer alignment for this approach, like maybe some panicing methods should gain non-panicing variants.