Lint tuple structs that could be named-field structs

As pointed out in rust-lang/rust#42998 it's an anti-pattern in Rust to create a tuple struct and then used named accessors, as in:

/// Type for Unicode Version.
#[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd)]
pub struct UnicodeVersion(
    pub u16, // Major version
    pub u16, // Minor version
    pub u16 // Micro (or Update) version
);

impl UnicodeVersion {
    /// Major version
    pub fn major(&self) -> u16 {
        self.0
    }

    /// Minor version
    pub fn minor(&self) -> u16 {
        self.1
    }

    /// Micro (or Update) version
    pub fn micro(&self) -> u16 {
        self.2
    }
}

We should lint cases like this and suggest replacing the tuple struct with a named-field struct, and adding a new method instead.

Start from the tuple-struct example in this issue and determine how Clippy should identify accessor methods that only expose positional fields. Done means the lint reports such cases, suggests a named-field struct and a new method, and handles the illustrated Rust pattern.