rethink the purpose of the `smallvec!` / `smallvec_inline!` macros

[alejandro-vaz]

these macros are essentially wrappers around the associated functions SmallVec::from and SmallVec::from_buf.

rust policy with macros, as written on the rust book, is simple: don't make a macro out of which can be made as a function

thus they seem very redundant

their origins trace to Vec API parity and a hack that had to be placed in 2021 in order to please the CTFE

conceptually:

smallvec![1u8, 2, 3] == SmallVec::from([1u8, 2, 3]);

smallvec_inline![1u8, 2, 3] == SmallVec::from_buf([1u8, 2, 3]);