Improve API and structure

[timsueberkrueb]

We should get rid of unnecessary attributes and use standard language features over macros whenever possible.

This is a rough sketch of what it could look like:

impl LALR for Parser {
    type T = Terminal;
    type N = Nonterminal;
    const START: Self::N = Nonterminal::Start;
    const END: Self::T = Terminal::End;
    type Input = str;
    type Output = f32;
}

#[lalr]
#[assoc(...)]
impl Parser { /* Rule functions ... */ }

where LALR is a trait defined as:

trait LALR {
    type T;
    type N;
    const START: Self::N;
    const END: Self::T;
    type Input;
    type Output;
}

[timsueberkrueb]

Looks like this would depend on rust-lang/rust#49683, unless there is a different way to refer to an enum variant over a type alias other than e.g. <Parser as LALR>::N::Variant.

[timsueberkrueb]

Update: Some steps I'm currently looking into:

• Change LALRLexer to Lexer
• Change LALRParser to Parser

because those traits are not LALR specific and the naming is confusing

• Generate start rule automatically
• Generate Nonterminal enum automatically

Those are currently unnecessary extra manual steps for the library user to maintain without providing any value. Therefore they should be generated.
Also, if we want to support precedence rules other than associativity (which is already supported) at some point in the future, we will need a way to define the precedence of a nonterminal in a way that is available at the time of the proc macro invocation. At this point, there is no proper way to extract information from an enum outside of the macro's scope.

[timsueberkrueb]

Done.