add accessor for StreamId u32

[ehaydenr]

No description provided.

[hawkw]

I think it seems pretty reasonable to add this. It is a public API commitment that a StreamId will always be convertible into a u32, but since the HTTP/2 standard defines stream IDs as being 31 bits, I think that's a pretty safe commitment for this library to make...

[LucioFranco]

Should it be a From impl or a get() -> u32?

[ehaydenr]

Doesn't make a huge difference to me. I find the From impl natural and this is exactly how it's done in the private StreamId type

h2/src/frame/stream_id.rs

Line 86 in 756e252

impl From<StreamId> for u32 {

Happy to change if desired.

[LucioFranco]

Personally, I don't like the hidden from statements but we could just add a get_u32 that is clear and verbose.