[Rprotobuf-yada] added some methods

[Romain François]

Hi,

I've added some methods today to the R interface.

- clear : to clear a message, or a field of a message
- swap : swap elements of a repeated field of a message
- size : number of elements of a repeated field
- [[ : which can index a message by field name or field tag number

and cleaned some of the mutator code : less copy and paste and slightly 
more efficient (I think).

I see that the proto library contains some unit test, so I'll base ours 
on it, at least I'll use the same messages, etc ...

Romain

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Romain Francois
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