Add helpful error message when detecting Rust style enums

[IGI-111]

Users moving from Rust might be tempted to use the Rust enum syntax:

enum A {
    Foo,
    Bar(u32),
}

When the correct Sway syntax is:

enum A {
    Foo: (),
    Bar: u32,
}

This is surprising, so we should have an error message that detects this pattern and asks the developer if they meant the latter.

[ironcev]

How the error message should look like? Let's use expressive diagnostics and have a detailed error similar to the one Rust is giving in this case:

enum E {
    A: (),
}

Error:

error: expected one of `(`, `,`, `=`, `{`, or `}`, found `:`
 --> src/main.rs:2:6
  |
1 | enum E {
  |      - while parsing this enum
2 |     A: (),
  |      ^ expected one of `(`, `,`, `=`, `{`, or `}`
  |
  = help: enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }`

Plus the additional Sway specific in-source help message saying, e.g. "Did you mean Foo: ()?" or "Did you mean Bar: u32?"

[jjcnn]

Plus the additional Sway specific in-source help message saying, e.g. "Did you mean Foo: ()?" or "Did you mean Bar: u32?"

This help message requires a way for ParseErrorKind to format a sway_ast::ty::Ty, and I can't to find a formatter for that type.

Is it worth the trouble to implement one, or should we instead go with something like "Enum kinds have the form 'Foo : ()' or 'Bar : u32'", without referring to the specific thing the user wrote?

[ironcev]

Indeed you are right, the type formatter is available further down in the type checking phase. I don't think that it's worth the trouble. Let's skip the specific in-source help and display just the general hint how enum variants can look like.