Add zero copy Message

[leshow]

#88 talked about no_std and a zero alloc parser for dhcp options. This does some initial work towards that. I put it in a new module borrowed under v4. It doesn't try to do everything that the owned version of Message does but it might be sufficient for most use cases. Internally it uses a Cow for options. The vast majority of options will not allocate, but in the case of a long-form option where the optcode and data have to be concatenated, it will return the Borrowed variant and allocate.

Still need some tests and a few more methods but I think this could be interesting for a zero-copy message type

[coveralls]

Pull Request Test Coverage Report for Build 18151245175

Details

• 232 of 258 (89.92%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+3.6%) to 51.277%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/v4/mod.rs	12	13	92.31%
src/v4/borrowed.rs	216	241	89.63%

Totals
Change from base Build 17251797289:	3.6%
Covered Lines:	1867
Relevant Lines:	3641

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💛 - Coveralls