Callable Syntax

[maxfischer2781]

This PR adds support for (A) -> R callable syntax. Changes include:

• [x] added Callable, Parameter and Signature representations to IR,
• [x] extended grammar to read (A, ...) -> R style steno types matching Python 3.8,
• [x] extended grammar to read (...) -> R special case,
• [x] support all Callable cases with .backend.typing.normalize,
• [x] extended grammar to read Callable types,
• [x] added unit tests for new features,

The grammar and IR represents the entire Python 3.8 call grammar. This far exceeds what typing.Callable can represent, and will require #29 (or similar) to emit again. Since there is no distinction between Callable and Protocol in stenotype syntax itself, implementing a single IR/grammar straight away seemed less hacky.

[maxfischer2781]

As it turns out, typing.Callable is absolutely not like typing.Generic at all. Representing the former with the latter is a hack one way or another.

I've added an explicit Callable IR type, which vastly simplifies handling (i.e. normalize and unparse). We should already cover all valid Callable forms as steno types:

$ venv/bin/stenotype '(*_) -> R'
typing.Callable[..., R]
$ venv/bin/stenotype '(A, B, C) -> R'
typing.Callable[[A, B, C], R]
$ venv/bin/stenotype '(A, B, /, C) -> R'
typing.Callable[[A, B, C], R]

Note that we cannot parse Callable[..., R] and Callable[[A, B, C], R] yet. However, this should be doable due to typing.Callable being absolutely not like typing.Generic at all.