Add documentation for values

[leonardt]

This initializes the cheat sheet doc (inspired by https://github.com/freechipsproject/chisel-cheatsheet) with documentation on magma values. Note that I discovered a weird issue with how values currently work, which is included in the documentation. This should be fixed by the hwtypes branch, so instead of working on a patch (which requires a non trivial change to how values work), I think effort is best put into getting the hwtypes branch in which will fundamentally change how values work.

[coveralls]

Pull Request Test Coverage Report for Build 2180

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 74.645%

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Totals
Change from base Build 2178:	0.0%
Covered Lines:	8832
Relevant Lines:	11832

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 2180

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 74.645%

---

Totals
Change from base Build 2178:	0.0%
Covered Lines:	8832
Relevant Lines:	11832

---

💛 - Coveralls

[rsetaluri]

btw, is this a <= issue or a more general wire() issue?

[leonardt]

it's a wire issue (in the way the logic handles anonymous values)