docs(core): document coding style

docs/core/misc/codestyle.md

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+# Trezor Core coding style
+
+## Python coding style
+
+Run `make pystyle` from repository root to perform all style checks and auto-format
+where possible.
+
+### General style notes
+
+See rules for exceptions in the [Exceptions documentation](./exceptions.md).
+
+#### Type annotations
+
+We prefer Python 3.10 style annotations:
+
+* instead of `List[int]`, use `list[int]`, dtto for `Tuple`, `Dict` and `Set`
+* instead of `Optional[int]`, use `int | None`
+* instead of `Union[int, str]`, use `int | str`
+
+This also applies inside `if TYPE_CHECKING` branches.
+
+#### Type-checking imports
+
+At run-time, the `typing` module is not available. There is compile-time magic that
+removes all `from typing` imports and contents of `if TYPE_CHECKING` branches.
+
+It is important to put typing-only imports into `if TYPE_CHECKING`, to make sure that
+these modules are not needlessly pulled in at run-time.
+
+Due to the compile-time magic, it is always possible to put a `from typing` import
+at top level. The style for doing that are as follows:
+
+* If the module needs to import other modules, create type aliases, TypeVars or
+  Protocols, the only top-level import should be `TYPE_CHECKING`. Everything else
+  (including other items from `typing` module) should be imported in the `TYPE_CHECKING`
+  branch:
+  ```python
+  from typing import TYPE_CHECKING
+
+  if TYPE_CHECKING:
+      from typing import Any, TypeVar, Union
+      from trezor.messages import SomeMessage
+
+      TypeAlias = Union[int, str]
+      T = TypeVar("T")
+  ```
+* If the module _only_ needs items from `typing`, you should not create a
+  `TYPE_CHECKING` branch, and instead import all required items on top level:
+  ```python
+  from typing import Any, Iterator, Sequence
+  ```
+
+### Tools
+
+Configurations of specific Python style tools (`isort`, `flake8`, `pylint`) can be found
+in root [`setup.cfg`].
+
+[`setup.cfg`]: https://github.com/trezor/trezor-firmware/blob/master/setup.cfg
+
+#### Formatting
+
+We are auto-formatting code with `black` and use the [`black` code
+style](https://black.readthedocs.io/en/stable/the_black_code_style/index.html).
+
+We use `isort` to organize imports.
+
+#### Linting
+
+We use `flake8` lints, disabling only those that conflict with `black` code style.
+
+We use a select subset of `pylint` checks that are hard-enforced.
+
+#### Type checking
+
+We use `pyright` for type-checking. The codebase is fully type-checked, except for
+the Monero app (as of 2022-01).
+
+
+## C coding style
+
+Formatting is done by `clang-format`. We are using the [Google code
+style](https://google.github.io/styleguide/cppguide.html).
+
+Run `make cstyle` from repository root to auto-format.
+
+## Rust coding style
+
+Formatting is done by `rustfmt`. We are using the [Rust
+style](https://github.com/rust-dev-tools/fmt-rfcs/blob/master/guide/guide.md).
+
+Run `make ruststyle` from repository root to auto-format.