Fix ALPN resolver being collected while OpenSSL still holds it

[SeanTAllen]

SSLContext.alpn_set_resolver handed the resolver to OpenSSL as ALPN select callback data, and nothing on the Pony side kept it alive. The deeper problem is that a session never held the Pony SSLContext that made it — SSL._create only took the raw SSL_CTX pointer, which keeps the C context alive through a refcount but is invisible to the garbage collector. So the Pony context, and the resolver it holds, could be collected while a session made from it was still handshaking. OpenSSL reads the resolver pointer live from the SSL_CTX and calls into freed memory. The trigger is remote: a peer connecting drives a server handshake into the callback.

The fix is that a session now holds its SSLContext. The context stays alive as long as any session made from it, and the context holds the current resolver, so the resolver lives as long as anything can call it. That closes the original drop-the-context case and the two follow-on cases — setting the resolver after a session exists, and replacing it — in one move. There is no per-session resolver copy anymore; the context is the single place the resolver is held.

To hold the context, a session, which is iso and gets sent between actors, needs a sendable reference to it, and that reference has to be val rather than tag because the garbage collector does not trace a tag's fields. So client() and server() now take a val receiver, and alpn_set_resolver and the ALPNProtocolResolver interface take val. This also makes the dangerous orderings impossible to write: setting the resolver needs a mutable context, making a session needs an immutable one, and a context is frozen once, so all configuration happens before any session exists.

ALPNStandardProtocolResolver is already val. Callers configure a context and then make sessions from it, which is already the idiomatic recover val SSLContext .> ... end pattern; callers who held a mutable context across session creation now have to freeze it first.

Closes #69. Closes #83.