add a "well-known" Sized trait to chalk_solve

[nikomatsakis]

I think the chalk-solve crate ought to start encoding rules for "well-known" traits that require compiler support. A good place to start is the Sized trait. I'm opening this issue to remember the problem, it will be extended with mentoring instructions.

[nikomatsakis]

Blocked on #356

[nikomatsakis]

So I think what we want here has two parts:

• First of all, we want to extend the 'well formedness' rules for struct definitions -- structs are not permitted to have "unsized fields" except in specific locations (notably, only the final field may be unsized). (Furthermore, enums are not permitted to have to unsized members...at all?)
• Based on this, rustc's code for checking whether a struct type is sized is quite specialized (important since this is a very common check) and it looks only at the final field of a struct. This may even have been necessary to prevent some kinds of cycles, I have a dim memory of that.

Some links from rustc code:

• adt_sized_constraint, sized_constraint_for_ty -- these functons are kind of minimally documented, but the basic idea is that given a type T, they return a type U such that T: Sized if U: Sized.
• the code in rustc that checks that a struct/enum/union's fields are sized

[basil-cow]

I'll take this one

[basil-cow]

blocked on #368

[marmeladema]

I hope #413 will help with this issue.

[jackh726]

This is done :) existing types have this logic and new types get it added in