In-place encoding

[heroin-moose]

I have a product that transfers flatbuffers over ring buffer in a shared memory between processes. Every slot is fixed size, so I can safely build in there, processing multiple events at once in async fashion. Building in-place would eliminate copying from heap, saving me some CPU time, given that transferred payloads can be quite large (up to 512k).

I propose a new API:

Builder::from_slice();
Builder::try_finish();

The first function will create a builder backed by a slice that cannot grow. The second will check that no overflow happened during building, returning an error if it did. Checking for overflow at the final stage allows to keep existing setters API intact.