modularize macros

[alejandro-vaz]

take the two macro_rules! macros of smallvec on lib.rs: smallvec! and smallvec_inline! and move them into their own file macros.rs

then make sure lib.rs does not use those macros at all (they are deprecated), and if it does, replace smallvec! for SmallVec::from and smallvec_inline! for SmallVec::from_buf