[quicwg/base-drafts] Clearer pseudocode/TLS syntax specification (#1449)

[Robin Marx <[e-mail]>]

I am quite new to the whole RFC stuff, so I am sorry for using hopelessly wrong names for things. 

I had some trouble today understanding some of the pseudocode in the transport draft, more specifically at https://tools.ietf.org/html/draft-ietf-quic-transport-12#section-6.4 :

```
struct {
 TransportParameterId parameter;
 opaque value<0..2^16-1>;
} TransportParameter;

struct {
 select (Handshake.msg_type) {
	case client_hello:
	   QuicVersion initial_version;

	case encrypted_extensions:
	   QuicVersion negotiated_version;
	   QuicVersion supported_versions<4..2^8-4>;
 };
 TransportParameter parameters<22..2^16-1>;
} TransportParameters;
```

For example the 'supported_versions' field: is that supposed to be an array prefixed with a length? Current implementations seem to think so. The 'value' field above is a single value prefixed with a length. But then the 'parameters' field below is very similar in syntax, but is not prefixed by length in the implementations (or I'm reading some things wrong, always possible).

I asked @larseggert about this, and he secretly admitted "Nobody but ekr understands the TLS syntax. It’s a huge pain.". Even if I'm misinterpreting some things, an alternative syntax or at least a clear explanation would be welcome (if that is already in there somewhere, possibly link it in https://tools.ietf.org/html/draft-ietf-quic-transport-12#section-2.1?)

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