Add back declarative macro definition forms

Version 1.0 only comes with primitive syntax transformer forms which is obviously not great for everyday use. Some initial syntax thoughts for a more declarative form:

// pattern matching
syntax new = function (ctx) {
  return match (ctx) {
    case`new $ident $params` => #`$ident.create $params`
  }
}
syntax let = function (ctx) {
  return match (ctx) {
    case`let $ident = $init:expr $rest ...` => #`
      (function ($ident) {
        $rest ...
      })($init)
    `
  }
}

This should happen after #378 and import ... for macros lands so the matching can just be a library.