auto merge of #15773 : P1start/rust/style-lints, r=alexcrichton

This unifies the `non_snake_case_functions` and `uppercase_variables` lints into one lint, `non_snake_case`. It also now checks for non-snake-case modules. This also extends the non-camel-case types lint to check type parameters, and merges the `non_uppercase_pattern_statics` lint into the `non_uppercase_statics` lint.

Because the `uppercase_variables` lint is now part of the `non_snake_case` lint, all non-snake-case variables that start with lowercase characters (such as `fooBar`) will now trigger the `non_snake_case` lint.

New code should be updated to use the new `non_snake_case` lint instead of the previous `non_snake_case_functions` and `uppercase_variables` lints. All use of the `non_uppercase_pattern_statics` should be replaced with the `non_uppercase_statics` lint. Any code that previously contained non-snake-case module or variable names should be updated to use snake case names or disable the `non_snake_case` lint. Any code with non-camel-case type parameters should be changed to use camel case or disable the `non_camel_case_types` lint.

This also adds support for lint groups to the compiler. Lint groups are a way of grouping a number of lints together under one name. For example, this also defines a default lint for naming conventions, named `bad_style`. Writing `#[allow(bad_style)]` is equivalent to writing `#[allow(non_camel_case_types, non_snake_case, non_uppercase_statics)]`. These lint groups can also be defined as a compiler plugin using the new `Registry::register_lint_group` method.

This also adds two built-in lint groups, `bad_style` and `unused`. The contents of these groups can be seen by running `rustc -W help`.

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