Reserve class namespace for RPC definitions

Integration of RFC7950 action statement has shown that generating a @FunctionalInterface definition for the invocation is nicely composable with lambdas – i.e. much better than the RpcService interface we are generating these days.

As a first step towards a solution, reserve the SchemaTree identifier implied by rpc statement. This changes behaviour when we have:

grouping foo;

rpc foo;

Where we currently generate: 

• a Foo, representing the grouping
• FooInput and FooOutput for RPC's input and output

i.e. 'rpc foo' does not assert its entitlement to Foo as per being a schema tree statement – which means there is no conflict with the grouping. It does assert the corresponding package name (.foo.input), which is a potential secondary conflict.

With this change we will generate a Foo$G for the grouping and nothing will be generated for Foo, as that name now belongs to 'rpc foo' instead of 'grouping foo'.