Add support for optional fields within shapes (#76)

[mwildehahn]

• Add support for optional shape fields & nested shapes

• Simplify CodegenShapeMember

• Don't use public class properties

• Add support for CodegenShape_FUTURE

• Remove old codegen

• Use correct codegen file

• Prefer vec

• Don't accept old shape

[mwildehahn]

LMK if this wasn't what you were talking about.