Support Project Transform in Favor of "Formula" Transform

[mrblueblue]

The Project Transform will basically be the Formula Transform.

It will support string expressions as well as object type expressions.

Expressions can also be specified as an array.

{
  type: "project",
  expr: Array<string | Expression> | string | Expression,
  as?: Array<string> | string
}

{
  type: "project",
  expr: {
     type: "date_trunc",
     unit: "month",
     field: "tweet_time",
     as: "key0"
  }
}
// SELECT date_trunc(month, tweet_time) as key0

{
  type: "project"
  expr: ["conv(lon)", "conv(lat)", "lang", "followers"],
  as: ["x", "y", "size", "color"]
}
// SELECT conv(lon) as x, conv(lat) as y, lang as size, followers as color

[mrblueblue]

fixed by #20