feat: add Context to EncodeBuffer and DecodeBuffer

[tjjfvi]

This allows codecs to store state for a given encode/decode run:

class FooCtx {
  fooCounter = 123;
}
const $foo = createCodec({
  ...
  _encode(buffer, value){
    ...
    const ctx = buffer.context.get(FooCtx);
    ctx.fooCounter;
    ...
  },
  ...
});

This can be used to encode/decode recursive structures – in particular, this will allow us in Capi to decode FRAME metadata in a recursive structure, rather than with the indices.

[harrysolovay]

This is a very exciting one.