Create new protobuf API

Following on from the exploration done in #18, we're going to create a new API for end users of this library.

Tasks:

[x] Move current code from protobuf.core to protobuf.impl.flatland.*
[x] Improve the organization of the code that's moved to protobuf.impl.flatland.*
[x] Create a protocol in protobuf.core and pull in behaviours and records for extend from impl.*
[x] In protobuf.impl.flatland.core just have a FlatlandProtoBuf record and its corresponding behaviour def
[x] Have ProtoBuf implementation records take a compiled Java protobuf class (inner) as a field, e.g.,:
```
(->MyProtoBuf :protobuf-class Example$Person)
```
[x] Figure out a static method approach for schema rendering
[x] Display instance data when printing protobuf impl in the REPL
[x] Improve usability of protobuf impl with core seq and map functions
[x] Update the docs to reflect all these changes

clojusc / protobuf