Removed dependency names can no longer be reused.

[nventuro]

Fixes #202.

I considered leaving a sentinel value to signal a name had been taken, but both owner and address need to be obviously zeroed out, and stake can be 0 (if minimalStake is 0), so I opted for adding an extra field. This should have a minimal impact on gas costs, since used is packed alongside owner, emitting a single SSTORE/SLOAD opcode when accessing (this was verified experimentally).

I'm not 100% sure on the name (maybe nameTaken?), but this is an internal detail and not part of the API.

[facuspagnuolo]

An alternative could be to have a separate mapping to tell whether a given dependency name was used or not: mapping (string => bool) dependencyUsed
Then we can full delete the Dependency struct and check for that mapping when registering new ones. What do you think?

[nventuro]

The only benefit of splitting them that way would be (arguably) cleaner code, in that you'd later be able to modify Dependency and not have to worry about keeping remove up to date, but it would make both create and remove more expensive (4 stores instead of 3).

The cost increase is not that large, and those functions shouldn't be used that often anyway, so it might be a reasonable trade-off.

[facuspagnuolo]

LGTM!