rustfmt: set imports_granularity to Item to minimize conflicts

Summary:
To make the codebase a bit more consistent and reduce conflicts. Similar to
Java.

Note: The fbsource Rust lint does not respect this file so it cannot conflict
with the fbcode rustfmt config. That means a more preserving (for what to group
in `{}` and what not), less controversial (when to use `{}` grouping is
controversial) choice `imports_layout = "HorizontalVertical"` (like
[black](https://github.com/psf/black) in Python) cannot be used at present.

See also:

- rust-lang/rustfmt#3361
- rust-lang/rustfmt#3362
- https://fb.workplace.com/groups/rust.language/posts/6720351014680123/
- https://fb.workplace.com/groups/rust.language/posts/5429720200409884/

This adds the config without changing the files.

Reviewed By: yancouto

Differential Revision: D31746731

fbshipit-source-id: 8e171829fd53691a59bf3b80cdc500c0b3993ba5

Author: quark-zju

eden/scm/.rustfmt.toml

@@ -0,0 +1,12 @@
+# Inherit from fbcode root
+edition = "2018"
+merge_derives = false
+use_field_init_shorthand = true
+
+# Project specific config.
+# Note: fbcode rustfmt does not respect these configs! So they cannot conflict
+# with fbcode format options (i.e. run fbcode rustfmt and the result isn't
+# changing).
+
+# To minimize conflicts, and make things easier to grep.
+imports_granularity = "Item"