Rearrange the generics chapter

[ehuss]

This rearranges things so that there is a comprehensive "Generics" chapter under "Types" that covers the various aspects of generics. The general outline is:

• Generics moved out of "Items" and into "Types". Although generics are defined in items, it really belongs in the type system chapter, since generics themselves are not items.
• Moved where clauses to the "Trait and lifetime bounds" chapter because where clauses are more closely aligned with bounds.
• Created a "Generic args" section, and moved some of the generic argument rules there. I suspect this should get filled out with more rules since it is pretty light right now.

I considered splitting this into sub-chapters, but I don't think it is too terribly long right now. I just worry it will grow to be quite long in the future.

There's a little more reworking such as mentioned in #925 about reworking the "Generic functions", but I didn't feel like doing more extensive rewriting at this time.