Include rustfmt config as IDE tool workaround

When using the `Format Document` command in VSCode,
rust-analyzer ends up looking at the workspace root for
the configuration, rather than starting at the file and
recursively searching outwards, like one might expect.

Work around this by providing a copy of rustfmt.toml
at the root level.  Note that this should not have any
effect on individual projects which have their own
configurations, as tested on the command line with
"cargo +nightly fmt" run at either the root or individual
project levels.

rustfmt.toml

@@ -0,0 +1,33 @@
+# A lot of the styles we want can only be configured
+# using unstable features.
+#
+# If you have trouble running rustfmt, try "cargo +nightly fmt".
+unstable_features = true
+
+edition = "2018"
+fn_args_layout = "Vertical"
+format_code_in_doc_comments = true
+imports_layout = "Vertical"
+match_block_trailing_comma = true
+max_width = 80
+merge_imports = true
+normalize_comments = true
+normalize_doc_attributes = true
+overflow_delimited_expr = true
+reorder_impl_items = true
+struct_lit_single_line = false
+use_field_init_shorthand = true
+use_small_heuristics = "Off"
+use_try_shorthand = true
+wrap_comments = true
+
+# This one isn't released yet.
+#fn_call_width = 40
+
+# These are already set to the defaults, but they are things
+# we really want to make sure are set, just in case defaults change.
+reorder_imports = true
+reorder_modules = true
+space_after_colon = true
+space_before_colon = false
+tab_spaces = 4