Add ParseFromIP convenience function

[janik-cloudflare]

This is similar to ParseFromIPAddr, but parses a net.IP instead of
an net.IPNet. The mask size is 32 for IPv4, or 128 for IPv6.

---

Just a small function that I thought might be worth upstreaming. Thank you all for this module!

[coveralls]

Pull Request Test Coverage Report for Build 2396213698

• 10 of 13 (76.92%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 81.215%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
net.go	10	13	76.92%

Totals
Change from base Build 2064315933:	-0.3%
Covered Lines:	147
Relevant Lines:	181

---

💛 - Coveralls

[aaronfuj]

Thanks for the PR, looks good! Can you add in a couple test cases to net_test.go to exercise both the IPv4 and IPv6 paths? They could be as simple as the existing tests, but it would still be good to have the coverage

e.g. something like the following

patricia/net_test.go

Lines 84 to 98 in 1ac5d9e

	_, ipr, _ := net.ParseCIDR("2001:0db8:85a3:0000:0000:8a2e:0370:7334/16")
	v4IP, v6IP, err = ParseFromIPAddr(ipr)
	assert.NoError(t, err)
	assert.Nil(t, v4IP)
	assert.NotNil(t, v6IP)
	assert.Equal(t, uint(16), v6IP.Length)
	assert.Equal(t, uint64(0x2001000000000000), v6IP.Left)
	assert.Equal(t, uint64(0x0), v6IP.Right)
	_, ipr, _ = net.ParseCIDR("127.0.0.1/10")
	v4IP, v6IP, err = ParseFromIPAddr(ipr)
	assert.NoError(t, err)
	assert.NotNil(t, v4IP)
	assert.Equal(t, uint(10), v4IP.Length)
	assert.Nil(t, v6IP)

[janik-cloudflare]

Thanks for the quick review @aaronfuj, added some simple tests (similar to ParseIPFromAddr)

[aaronfuj]

Thanks for the quick review @aaronfuj, added some simple tests (similar to ParseIPFromAddr)

No problem, thanks for the PR!

[aaronfuj]

lgtm!