Improve explanation of Multi Parameter Type Classes and Functional Dependencies

[milesfrain]

I don't think beginners (without a Haskell background) are going to be able to absorb much from these sections:
https://book.purescript.org/chapter6.html#multi-parameter-type-classes
https://book.purescript.org/chapter6.html#functional-dependencies

It would be more appropriate to cover this material in the language reference (and as a more thorough guide too), and then link to this material from the book. It's currently inverted, where the docs repo links to the book instead:
https://github.com/purescript/documentation/blob/master/language/Type-Classes.md#multi-parameter-type-classes
https://github.com/purescript/documentation/blob/master/language/Type-Classes.md#functional-dependencies

Exercises for this content also need to be added to the book.

This material didn't make much sense for me until watching this video.
https://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Dr-Ralf-Lmmel-Advanced-Functional-Programming-Type-Classes

Timestamps for Functional Dependencies are:

• 29:13 - setup scenario
• 34:20 - describe problem
• 38:35 - functional dependencies as the solution