Remove scope method from the provider interface

[ghost]

@peter-edge mentioned, that using Scope in Provider is very clumsy and we actually don't really need every Provider to implement Scope method, Get should be enough. Users can either use ScopedProvider to create a new scope for a provider or we can add this functionality to the Value. Users can simply call:
config.Get("yarpc").Get("http").Get("port")
instead of
config.Scope("yarpc").Scope("http").Get("port")

[coveralls]

Coverage decreased (-0.1%) to 91.276% when pulling a33fe48 on alsam-removeConfigScope into d171cfb on alsam-ifaceConfig.

[ascandella]

Huh. Did Scope() used to do something else? Or was it always equivalent to Get()?

[ascandella]

Maybe mention why briefly? "because it was unused" or whatever

[ascandella]

Also this is potentially [BREAKING] right, since people could be calling Scope() in their code?

[coveralls]

Coverage decreased (-0.1%) to 91.273% when pulling 6e9a162 on alsam-removeConfigScope into d171cfb on alsam-ifaceConfig.

[ghost]

Scope supposed to return a provider that appends a prefix to the current provider. We implemented it for every provider differently, but I've changed to use the scoped provider underneath everywhere a couple month ago, and with addition of Get method to Value, we can simply remove the repeated code and method from the interface.

[madhuravi]

Nice!! Scope always bothered me here, Get is much nicer.

[coveralls]

Coverage decreased (-0.1%) to 91.229% when pulling 8c59350 on alsam-removeConfigScope into 2c88d6f on master.