Use rustfmt to enforce consistent formatting

This change adds a .rustfmt.toml that includes ALL supported settings,
12 of which we have overridden to attempt to cater to our own
proclivities.

rustfmt is checked in the rust-nightly CI job.

.rustfmt.toml

@@ -0,0 +1,84 @@
+# https://github.com/rust-lang-nursery/rustfmt/blob/master/Configurations.md
+verbose = false
+disable_all_formatting = false
+skip_children = false
+max_width = 100
+error_on_line_overflow = false
+error_on_line_overflow_comments = false
+tab_spaces = 4
+fn_call_width = 60
+struct_lit_width = 18
+struct_variant_width = 35
+force_explicit_abi = true
+newline_style = "Unix"
+fn_brace_style = "SameLineWhere"
+item_brace_style = "SameLineWhere"
+control_style = "Rfc"
+control_brace_style = "AlwaysSameLine"
+impl_empty_single_line = true
+trailing_comma = "Vertical"
+trailing_semicolon = true
+fn_empty_single_line = true
+fn_single_line = false
+fn_return_indent = "WithArgs"
+fn_args_paren_newline = false
+fn_args_density = "Tall"
+fn_args_layout = "Block"
+array_layout = "Block"
+array_width = 60
+array_horizontal_layout_threshold = 0
+type_punctuation_density = "Wide"
+where_style = "Rfc"
+where_density = "CompressedIfEmpty"
+where_layout = "Vertical"
+where_pred_indent = "Visual"
+generics_indent = "Block"
+struct_lit_style = "Block"
+struct_lit_multiline_style = "ForceMulti"
+fn_call_style = "Visual"
+report_todo = "Never"
+report_fixme = "Never"
+chain_indent = "Block"
+chain_one_line_max = 60
+chain_split_single_child = false
+imports_indent = "Block"
+imports_layout = "HorizontalVertical"
+reorder_extern_crates = true
+reorder_extern_crates_in_group = true
+reorder_imports = true
+reorder_imports_in_group = true
+reorder_imported_names = true
+single_line_if_else_max_width = 50
+format_strings = true
+force_format_strings = false
+take_source_hints = false
+hard_tabs = false
+wrap_comments = false
+comment_width = 80
+normalize_comments = false
+wrap_match_arms = true
+match_block_trailing_comma = false
+indent_match_arms = true
+match_pattern_separator_break_point = "Back"
+closure_block_indent_threshold = 0
+space_before_type_annotation = false
+space_after_type_annotation_colon = true
+space_before_struct_lit_field_colon = false
+space_after_struct_lit_field_colon = true
+space_before_bound = false
+space_after_bound_colon = true
+spaces_around_ranges = false
+spaces_within_angle_brackets = false
+spaces_within_square_brackets = false
+spaces_within_parens = false
+use_try_shorthand = true
+write_mode = "Overwrite"
+condense_wildcard_suffixes = false
+combine_control_expr = true
+struct_field_align_threshold = 0
+remove_blank_lines_at_start_or_end_of_block = true
+attributes_on_same_line_as_field = true
+attributes_on_same_line_as_variant = true
+multiline_closure_forces_block = false
+multiline_match_arm_forces_block = false
+merge_derives = true