refactor: Replace byteorder with std functions

[cronokirby]

Fixes #388.

This favors the x::from_yy_bytes functions instead of using the byteorder crate. This crate is removed as a dependency.

There was a single use of NativeEndian from byteorder, when generating random numbers. This was replaced with a use of from_be_bytes instead. This has no semantic differences, but might be a bit slower on little endian platforms.

The changes were otherwise straightforward.

[coveralls]

Pull Request Test Coverage Report for Build 2571

• 6 of 6 (100.0%) changed or added relevant lines in 3 files are covered.
• 4 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.02%) to 96.275%

---

Files with Coverage Reduction	New Missed Lines	%
src/toxcore/dht/server/mod.rs	1	95.12%
src/toxcore/tcp/server/server_ext.rs	1	84.96%
src/toxencryptsave/mod.rs	2	91.84%

Totals
Change from base Build 2552:	-0.02%
Covered Lines:	18894
Relevant Lines:	19625

---

💛 - Coveralls