refactor(rand): use rand crate instead of sodiumoxide

[coveralls]

Pull Request Test Coverage Report for Build 782118931

• 192 of 199 (96.48%) changed or added relevant lines in 25 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 95.05%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tox_packet/src/ip_port.rs	13	14	92.86%
tox_core/src/onion/client/mod.rs	26	28	92.86%
tox_core/src/dht/server/mod.rs	42	46	91.3%
<!--	Total:	192	199	96.48%	-->

Totals
Change from base Build 491807937:	0.02%
Covered Lines:	16052
Relevant Lines:	16888

---

💛 - Coveralls