Enable implicit_return and fix all violations

[mildm8nnered]

The only changes here are

1. Enabling (or not disabling) implicit_return
2. Fix all existing 300+ violations
3. Whitespace adjustments for parameter alignment where removing return threw things out.

[SwiftLintBot]

	2 Warnings
⚠️	Big PR
⚠️	If this is a user-facing change, please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.

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Here's an example of your CHANGELOG entry:

* Enable `implicit_return` and fix all violations.  
  [mildm8nnered](https://github.com/mildm8nnered)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

note: There are two invisible spaces after the entry's text.

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[SimplyDanny]

3. Whitespace adjustments for parameter alignment where removing return threw things out.

Can you give an example for such an adjustment? Is it something that the rewriter could do better?

[mildm8nnered]

5. Whitespace adjustments for parameter alignment where removing return threw things out.

Can you give an example for such an adjustment? Is it something that the rewriter could do better?

So I didn't actually use the rewriter, because I didn't spot that the rule subclassed SwiftSyntaxCorrectableRule 🤦, so much so that I even thought "Hey, I could make this rewritable while I'm here".

Trying the rewriter, it actually has a few problems:

In YamlParser.swift

    static func constructExpandingEnvVars(env: [String: String]) -> (_ scalar: Node.Scalar) -> String? {
        return { (scalar: Node.Scalar) -> String? in
            return scalar.string.expandingEnvVars(env: env)
        }
    }

becomes

    static func constructExpandingEnvVars(env: [String: String]) -> (_ scalar: Node.Scalar) -> String? {
        { (scalar: Node.Scalar) -> String? in
            return string.expandingEnvVars(env: env)
        }
    }

which fails to compile with

/Users/martin.redington/Documents/Source/SwiftLint/Source/SwiftLintCore/Models/YamlParser.swift:44:20: error: cannot find 'string' in scope
            return string.expandingEnvVars(env: env)

Manually I had

    static func constructExpandingEnvVars(env: [String: String]) -> (_ scalar: Node.Scalar) -> String? {
        { (scalar: Node.Scalar) -> String? in
            scalar.string.expandingEnvVars(env: env)
        }
    }

Rolling through the autocorrects, it looks like when we have nested implicit returns, that causes problems.

I haven't looked into what's going on, but I suspect that the offsets for the nested implicit return are off once the outer one has been corrected.

The whitespace issues were actually much more minor. In a case like SyntacticSugarRule

            return StyleViolation(ruleDescription: Self.description,
                                  severity: configuration.severity,
                                  location: Location(file: file, byteOffset: ByteCount(violation.position)),
                                  reason: violation.type.violationReason)

corrected to

            StyleViolation(ruleDescription: Self.description,
                                  severity: configuration.severity,
                                  location: Location(file: file, byteOffset: ByteCount(violation.position)),
                                  reason: violation.type.violationReason)

This will then violate one of the parameter alignment rules.

I'll write up the Rewriter issues separately - it would be nice to fix that, as this is a rule that people probably have a lot of violations of, and doing them by hand is probably a pain for most people.

Something I also missed running the auto-correction was the ability to specify a single rule to enable on the command line. Please autocorrect just this one rule - unless I'm missing something and we can already do that.