Immutable QueryParams

[lovelydinosaur]

As outlined in #1599.
Refs #1275.

Drop support for QueryParams.__setitem__ and QueryParams.update() in favour of an immutable-only interface, in line with our URL design. All manipulation methods now return new QueryParams instances:

q=httpx.QueryParams()
q=q.set("a", "123")
assertq==httpx.QueryParams("a=123")
q=q.add("a", "456")
assertq==httpx.QueryParams("a=123&a=456")
q=q.remove("a")
assertq==httpx.QueryParams("")
q=q.merge({"b": "789"})
assertq==httpx.QueryParams("b=789")

I've opted for a hard deprecation here, with .update() raising a RuntimeError, because immutability vs. mutability isn't something we ought to go "we're halfway there" on.

[lovelydinosaur]

I'm pushing on with this because it's something I've already been considering for a long time, and I've got other stuff that it's going to block unless I just push on with it, so...