Improving instance __repr__ cases.

[tomchristie]

Connection pools are currently missing a __repr__.

We should expose how many connections are open, and how many active/queued requests are being handled...

<ConnectionPool [0 connections, 0 active requests, 0 queued requests]>
<HTTPProxy "http://localhost:8080" [0 connections, 0 active requests, 0 queued requests]>
<SOCKSProxy "http://localhost:8080" [0 connections, 0 active requests, 0 queued requests]>

Response byte streams could include __repr__ info...

>>> with httpcore.stream("GET", "https://www.example.com") as response:
...     print(response)
...     print(response.stream)
...     response.read()
...     print(response.stream)
<Response [200]>
<ByteStream [0 bytes read, OPEN]>
<ByteStream [1256 bytes read, CLOSED]>

These would be implemented on HTTP11ConnectionByteStream and HTTP2ConnectionByteStream. The ConnectionPoolByteStream would need __repr__ to proxy onto those classes.

Network streams could include __rep__ info...

>>> with httpcore.stream("GET", "https://www.example.com") as response:
...     print(response.extensions["network_stream"])
<SyncNetworkStream [('93.184.216.34', 443), SSL, 0 bytes read, 2048 bytes written, OPEN]>

The __repr__ for request instances could be improved, instead of this...

>>> httpcore.Request("GET", "https://www.example.com")
<Request [b'GET']>

I'd rather see this...

>>> httpcore.Request("GET", "https://www.example.com")
<Request ['GET', 'https://www.example.com']>

If anyone is interested in taking these on I'd suggest opening an issue for one of the above cases, and we could work through from there.