Add rustfmt config file

[liss-h]

The following content should be sufficient for now:

imports_granularity = "Crate"
max_width = 120
struct_lit_width = 70
match_block_trailing_comma = true

It should be automatically downloaded for all our projects using rust.

I've looked up where the file is supposed to be placed (TLDR same place where we put the clang files): "You can create a TOML file called .rustfmt.toml, place it in the project or any other parent directory and it will apply the options in that file."

[bigerl]

Question: how to integrate it into the repos:

• tentris-lib: cmake
• tentris-frontend: build script?