[eval] add -D eval-pretty-print

[kLabz]

Adds -D eval-pretty-print to enable basic pretty printing of eval values (indenting on object fields).

Example output with -D eval-print-depth=10 -D eval-pretty-print

Main.hx:6: {
  pos: #pos(Main.hx:6: lines 6-8),
  expr: EBlock([{
    pos: #pos(Main.hx:7: characters 3-37),
    expr: ECall({
      pos: #pos(Main.hx:7: characters 3-26),
      expr: EField({
        pos: #pos(Main.hx:7: characters 3-21),
        expr: EField({
          pos: #pos(Main.hx:7: characters 3-17),
          expr: EField(<...>,<...>,<...>)
        },baz,Normal)
      },test,Normal)
    },[{
      pos: #pos(Main.hx:7: characters 27-36),
      expr: EArrayDecl([{
        pos: #pos(Main.hx:7: characters 28-29),
        expr: EConst(<...>)
      },{
        pos: #pos(Main.hx:7: characters 31-32),
        expr: EConst(<...>)
      },{
        pos: #pos(Main.hx:7: characters 34-35),
        expr: EConst(<...>)
      }])
    }])
  }])
}

Instead of:

Main.hx:6: {pos: #pos(Main.hx:6: lines 6-8), expr: EBlock([{pos: #pos(Main.hx:7: characters 3-37), expr: ECall({pos: #pos(Main.hx:7: characters 3-26), expr: EField({pos: #pos(Main.hx:7: characters 3-21), expr: EField({pos: #pos(Main.hx:7: characters 3-17), expr: EField(<...>,<...>,<...>)},baz,Normal)},test,Normal)},[{pos: #pos(Main.hx:7: characters 27-36), expr: EArrayDecl([{pos: #pos(Main.hx:7: characters 28-29), expr: EConst(<...>)},{pos: #pos(Main.hx:7: characters 31-32), expr: EConst(<...>)},{pos: #pos(Main.hx:7: characters 34-35), expr: EConst(<...>)}])}])}])}

It also supports an optional value setting a custom indentation string, but I didn't document it because at the moment it doesn't work well with \t (printed as-is instead of printing a tab character) which I expect people to try out..