Better integrate cmake example

[franziskuskiefer]

• Describe how to use HACL in a CMake project in the book.
• Run the cmake example on CI

No windows in CI for now (it's failing and needs debugging).

Fixes #268

[coveralls]

Pull Request Test Coverage Report for Build 3702312332

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 55.966%

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Totals
Change from base Build 3697687686:	0.0%
Covered Lines:	33754
Relevant Lines:	60312

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💛 - Coveralls

[duesee]

Could you file an issue about the Windows CI failure?