default keyword for layout definition

[zebraNeon]

currently for type B = { f1: #{ f1: U8, f2: U8 }, f2: U16 } layout record { f1: default, f2: 2B after f1 }, it will generate record { f1: record { f1: 1B, f2: 1B after f1 }, f2: 2B after f1 }