Introduce rustfmt config file

As we did in `rust-bitcoin` introduced a `rustfmt` configuration file
that is palatable to devs.

Do not run formatter, that is done as a separate patch to assist review.

Author: tcharding

rustfmt.toml

@@ -1 +1,82 @@
-disable_all_formatting = true
+# Eventually this shoud be: ignore = []
+ignore = [
+       "secp256k1-sys"
+]
+
+hard_tabs = false
+tab_spaces = 4
+newline_style = "Auto"
+indent_style = "Block"
+
+max_width = 100 # This is number of characters.
+# `use_small_heuristics` is ignored if the granular width config values are explicitly set.
+use_small_heuristics = "Max"    # "Max" == All granular width settings same as `max_width`.
+# # Granular width configuration settings. These are percentages of `max_width`.
+# fn_call_width = 60
+# attr_fn_like_width = 70
+# struct_lit_width = 18
+# struct_variant_width = 35
+# array_width = 60
+# chain_width = 60
+# single_line_if_else_max_width = 50
+
+wrap_comments = false
+format_code_in_doc_comments = false
+comment_width = 100             # Default 80
+normalize_comments = false
+normalize_doc_attributes = false
+format_strings = false
+format_macro_matchers = false
+format_macro_bodies = true
+hex_literal_case = "Preserve"
+empty_item_single_line = true
+struct_lit_single_line = true
+fn_single_line = true           # Default false
+where_single_line = false
+imports_indent = "Block"
+imports_layout = "Mixed"
+imports_granularity = "Module"     # Default "Preserve"
+group_imports = "StdExternalCrate" # Default "Preserve"
+reorder_imports = true
+reorder_modules = true
+reorder_impl_items = false
+type_punctuation_density = "Wide"
+space_before_colon = false
+space_after_colon = true
+spaces_around_ranges = false
+binop_separator = "Front"
+remove_nested_parens = true
+combine_control_expr = true
+overflow_delimited_expr = false
+struct_field_align_threshold = 0
+enum_discrim_align_threshold = 0
+match_arm_blocks = false        # Default true
+match_arm_leading_pipes = "Never"
+force_multiline_blocks = false
+fn_args_layout = "Tall"
+brace_style = "SameLineWhere"
+control_brace_style = "AlwaysSameLine"
+trailing_semicolon = true
+trailing_comma = "Vertical"
+match_block_trailing_comma = false
+blank_lines_upper_bound = 1
+blank_lines_lower_bound = 0
+edition = "2018"
+version = "One"
+inline_attribute_width = 0
+format_generated_files = true
+merge_derives = true
+use_try_shorthand = false
+use_field_init_shorthand = false
+force_explicit_abi = true
+condense_wildcard_suffixes = false
+color = "Auto"
+required_version = "1.5.1"
+unstable_features = false
+disable_all_formatting = false
+skip_children = false
+hide_parse_errors = false
+error_on_line_overflow = false
+error_on_unformatted = false
+emit_mode = "Files"
+make_backup = false