Add a procedural macro for defining layouts

[Skelebot]

Add a procedural macro to generate keyberon::layout::Layers

• Solves keyboard layout prettyness #49
• In future versions const fns to create Actions could be deprecated/removed, solving avoid relying on implicit promotion #41

Possible future extensions

• Compiler warnings when rows have inconsistent number of keycodes / layers have inconsistent number of rows
• Additional layout setup like automatic impl_heterogeneous_map (solving Clean impl_heterogenous_map #16)

Issues

• Does not differentiate between Action::MultipleKeyCodes and Action::MultipleActions, so the macro always uses MultipleActions which can increase binary size slightly (possible improvement in the future)
• Commas can accidentaly be used and parsed as a key (possibly solved by compiler warnings on inconsistent number of keycodes in rows)

Syntax

Items inside the macro are converted to Actions as such:

• Action::KeyCode: Idents are automatically understood as keycodes: A, RCtrl, Space
  • Punctuation, numbers and other literals that aren't special to the rust parser are converted
    to KeyCodes as well: , becomes KeyCode::Commma, 2 becomes KeyCode::Kb2, / becomes KeyCode::Slash
  • Characters which require shifted keys are converted to Action::MultipleKeyCodes(&[LShift, <character>]):
    ! becomes Action::MultipleKeyCodes(&[LShift, Kb1]) etc
  • Characters special to the rust parser (parentheses, brackets, braces, quotes, apostrophes, underscores, backslashes and backticks)
    left alone cause parsing errors and as such have to be enclosed by apostrophes: '[' becomes KeyCode::LBracket,
    '\'' becomes KeyCode::Quote, '\\' becomes KeyCode::BSlash
• Action::NoOp: Lowercase n
• Action::Trans: Lowercase t
• Action::Layer: A number in parentheses: (1), (4 - 2), (0x4u8 as usize)
• Action::MultipleActions: Actions in brackets: [LCtrl S], [LAlt LCtrl C], [(2) B {Action::NoOp}]
• Other Actions: anything in braces ({}) is copied unchanged to the final layout - { Action::Custom(42) }
  simply becomes Action::Custom(42)

Important note: comma (,) is a keycode on its own, and can't be used to separate keycodes as one would have
to do when not using a macro.

Usage example:

Example layout for a 4x12 split keyboard:

use keyberon::action::Action;
static DLAYER: Action = Action::DefaultLayer(5);

pub static LAYERS: keyberon::layout::Layers = keyberon::layout::layout! {
    {
        [ Tab    Q W E R T   Y U I O P BSpace ]
        [ LCtrl  A S D F G   H J K L ; Quote  ]
        [ LShift Z X C V B   N M , . / Escape ]
        [ n n LGui {DLAYER} Space Escape   BSpace Enter (1) RAlt n n ]
    }
    {
        [ Tab    1 2 3 4 5   6 7 8 9 0 BSpace  ]
        [ LCtrl  ! @ # $ %   ^ & * '(' ')' - = ]
        [ LShift n n n n n   n n n n n [LAlt A]]
        [ n n LGui (2) t t   t t t RAlt n n    ]
    }
    // ...
};

[theotherjimmy]

Looks, and works, great!

[TeXitoi]

Thanks!

You also need to add an entry in the changelog

[TeXitoi]

Thanks

[gilescope]

slick!