feature: cargo-msrv suggest

[foresterre]

{
    type: "suggestion",
    suggestion: 'increment-msrv',
    new_msrv: '1.58'
    reasons: [
        { reason: 'date-rule', 'description': 'The MSRV is defined as the latest release, released at least 1 year ago', source: { type: 'file', path: 'Cargo.toml', key: 'package.metadata.msrv.rules.date' } }
    ]
}

{
    type: "suggestion",
    suggestion: 'increment-msrv',
    new_msrv: '1.58'
    reasons: [
        { reason: 'last-minus-n-rule', 'description': 'The MSRV is defined as the latest release - 2', source: { type: 'file', path: 'Cargo.toml', key: 'package.metadata.msrv.rules.date' } }
    ]
}

cargo msrv suggest --apply

cargo msrv suggest --include "increment-msrv" --exclude "x"

cargo msrv suggest --export suggestions.json
cargo msrv suggest --import suggestions.json

See also: #281
Probably hould be preceded by: #725