Use rustfmt for code formatting

[TheDan64]

Is your feature request related to a problem? Please describe.
@longwusha had a great suggestion - we should have code styling preferences. Particularly via rustfmt.

Describe the solution you'd like
We should use rustfmt to standardize code style.

Describe possible drawbacks to your solution
My ideal configuration for rustfmt uses unstable options, but this project is on stable. So I do not wish to enforce nightly rustfmt.

Additional context
Stabilization checklist WIP:

• comment_width = 100 - [unstable option] comment_width rust-lang/rustfmt#3349
• format_doc_comments = true - [unstable option] format_code_in_doc_comments rust-lang/rustfmt#3348
• match_arm_blocks = false - [unstable option] match_arm_blocks rust-lang/rustfmt#3373
• match_block_trailing_comma = true - [unstable option] match_block_trailing_comma rust-lang/rustfmt#3380
• max_width = 120
• merge_derives = true
• merge_imports = true - [unstable option] merge_imports rust-lang/rustfmt#3362
• newline_style = "Unix"
• use_field_init_shorthand = true
• use_try_shorthand = true
• wrap_comments = true - [unstable option] wrap_comments rust-lang/rustfmt#3347

[TheDan64]

Blocked until rustfmt v2 which will let you use unstable features on stable

[TheDan64]

Going to add the rustfmt settings that are available now & we can add more later