Implementing field mask as a set (some map implementation), rather than an array.

[akashagrahari-mt]

It is frequently required to check whether a specific path exists in the list of field mask paths. Implementing it as a set would make it a lot easier.

[dsnet]

Closing as duplicate of #398.

The underlying representation can’t be a map since it must be a faithful representation of what field_mask.proto specifies. That said we intend to provide functionality to make it easier to interact with field mask.