Add MacroExp for freestanding macro expressions

[leogdion]

Summary

SyntaxKit has no support for freestanding macro expressions (MacroExpansionExprSyntax). A new MacroExp type should be added to cover #macroName(args...) syntax.

Details

• Conforms to ExprCodeBlock (expression, not a declaration)
• Follows the Exp suffix naming convention used by FunctionCallExp, PropertyAccessExp, etc.
• Maps to MacroExpansionExprSyntax in SwiftSyntax
• Covers both built-in macros and custom user-defined macros:
  • #stringify(x)
  • #colorLiteral(red:green:blue:alpha:)
  • #imageLiteral(resourceName:)
  • Any custom #myMacro(arg:) defined via @freestanding(.expression)

Example API

MacroExp("stringify", ReferenceExp("x"))
// → #stringify(x)

MacroExp("colorLiteral") {
  Argument("red", Literal.float(0.5))
  Argument("green", Literal.float(0.2))
  Argument("blue", Literal.float(0.8))
  Argument("alpha", Literal.float(1.0))
}
// → #colorLiteral(red: 0.5, green: 0.2, blue: 0.8, alpha: 1.0)