Improve errors when keywords are used as identifiers

[tertsdiepraam]

Closes #153.

That example will now fail with this message:

thread 'main' panicked at examples/foo.rs:12:5:
called `Result::unwrap()` on an `Err` value: "Name \"accept\" is a keyword in Roto and therefore not a valid identifier"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The parser will also report more useful errors when a keyword is used when an identifier is expected, the difference is the note at the end of the error:

Error: Parse error: expected an identifier but got 'accept'
   ╭─[examples/simple.roto:2:10]
   │
 2 │ function accept(x: IpAddr) -> bool {
   │          ───┬──  
   │             ╰──── expected an identifier
   │ 
   │ Note: `accept` is a keyword and cannot be used as an identifier.
───╯

and the other case where this happens:

Error: Parse error: expected an identifier, `super`, `pkg` or `dep` but got 'accept'
   ╭─[examples/simple.roto:1:8]
   │
 1 │ import accept;
   │        ───┬──  
   │           ╰──── expected an identifier, `super`, `pkg` or `dep`
   │ 
   │ Note: `accept` is a keyword and cannot be used as an identifier.
───╯

The superfluous ` mark has also been removed.

[bal-e]

Looks good! I'm a bit surprised just how much stuff this relatively simple refactor involved.

[bal-e]

Neat!

[bal-e]

Nit: maybe have fn Keyword::code(&self) -> &'static str and use that here? It might come in handy in a few other situations.

[tertsdiepraam]

Yeah probably, although I don't like code. Maybe as_str?

[tertsdiepraam]

Looks good! I'm a bit surprised just how much stuff this relatively simple refactor involved.

Yeah well it's a couple things I suppose:

• Splitting keywords from other tokens
• Move the runtime tests
• Error on doing this wrong in a Roto script
• Error on doing this wrong while registering a function

So all in all it's not trivial.