Partial implementation of the parsing of Parcel into Metadata.

In this first cut, a raw parcel is parsed to check that the overall
format is correct.

At the same time, we record the metadata seen and their position in
the parcel for later retrieval using the get* methods.

This means that the 'has' method to check the existence of a metadata
should work.

Removed size and iterator methods. Instead, I added a new method
to give access to the set of keys. The user can make use of the set
to call size(), empty(), iterator() etc...