add `rustfmt.toml` with style guidelines

[alejandro-vaz]

IMHO cargo fmt is a pretty good tool, not perfect but pretty good, it is configurable to your taste and coding style, try experimenting with rustfmt.toml, my favorite setting is:

wrap_comments = true
imports_granularity = "Crate"
group_imports = "StdExternalCrate"
max_width = 80
format_code_in_doc_comments = true

But that's my taste. you might not like it.
In the end rust is using and enforcing fmt itself, I think if it is good enough for rust it would be good enough for smallvec.
It would be nice to keep the same principles as rust project, as rust project itself depends on smallvec.

Breaking formatting too much solution

• run a cargo fmt once commit the changes to @ghost, it is a future proof solution to a current problem.

Originally posted by @fereidani in #429

Important

both v1 and v2