Expand URL interface

[tomchristie]

Neaten out the URL interface as follows:

• Support httpx.URL(**kwargs)
• Support url.copy_with(params=...)
• Support url.params
• Support url.copy_set_param(), url.copy_add_param(), url.copy_remove_param(), url.copy_merge_params()

For instance:

u = httpx.URL("https://www.example.com/", username="tom@gmail.com", password="123 456")
assert u == "https://tom%40gmail.com:123%20456@www.example.com/"

u = httpx.URL(scheme="https", host="www.example.com", path="/')
assert u == "https://www.example.com/"

u = httpx.URL("https://www.example.com/")
u = u.copy_with(params={"search": "is green a colour?"}
assert u == "https://www.example.com/?search=is+green+a+colour%3F"

u = httpx.URL("https://www.example.com/?a=123")
assert u.params == httpx.QueryParams({"a": "123"})

u = httpx.URL("https://www.example.com/?a=123")
u = u.copy_set_param("a", "456")
assert u == "https://www.example.com/?a=456"

u = httpx.URL("https://www.example.com/?a=123")
u = u.copy_add_param("a", "456")
assert u == "https://www.example.com/?a=123&a=456"

u = httpx.URL("https://www.example.com/?a=123")
u = u.copy_remove_param("a")
assert u == "https://www.example.com/"

u = httpx.URL("https://www.example.com/?a=123")
u = u.copy_merge_params({"b": "456"})
assert u == "https://www.example.com/?a=123&b=456"

There's one edge case bit of necessary straightening out here, which is the behaviour of:

u = httpx.URL("https://www.example.com/?a=123", params={"b": "456"})

Which now replaces the params, rather than merging them. Because that's the consistent thing to do if params is just one of many possible keyword arguments there, and should get consistent behaviour.

Note however that I've continued to ensure that #652 remains resolved. So:

httpx.get("https://www.example.com/?a=123", params={"b": "456"})  # Outgoing request uses "?a=123&b=456" here.

# Is equivalent to...
url = httpx.URL("https://www.example.com/?a=123")
url = url.copy_merge_params({"b": "456"}
httpx.get(url)

[tomchristie]

Righty, this is the last set of stuff I wanted to get into 0.18. Planning to merge this tomorrow and then release.

Full release notes at #1576