add Default trait to generated types

[gbonnefille]

Default trait is useful when creating proof-of-concept tools.

Fixes #146

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[gbonnefille]

@hchataing feel free to create a feature inspired by this patch (I'm not able to accept the CLA)

[hchataing]

Patch rebased to #147 (that was a failed experiment to see if me uploading the patch would bypass the CLA enforcement)

[gbonnefille]

@hchataing thinking about this feature, I realized we can certainly go one step further looking at Builder pattern.
For example, adding methods like with_XXX(YYY) and then being able to write:

let packet = MyPacket::default().with_attr1(1).with_attr3(3);

I tried to implement this, but I'm still not comfortable enough with the code.

[hchataing]

@hchataing thinking about this feature, I realized we can certainly go one step further looking at Builder pattern. For example, adding methods like with_XXX(YYY) and then being able to write:

let packet = MyPacket::default().with_attr1(1).with_attr3(3);

I tried to implement this, but I'm still not comfortable enough with the code.

You could use this pattern to specify a subset of fields only:

MyPacket { MyPacket::default() .., attr1: 1, attr3: 3 }

I haven't seen a lot of use cases for using the with_attr() setters, in what context do you want to have this ?

[gbonnefille]

I did not know the pattern you presented. So there is no more need. Thanks.