Use the slice repr for ~[T]
Use temp vars for implicit coercion to ^[T]
DST coercions and DST structs

[breaking-change]

1. The internal layout for traits has changed from (vtable, data) to (data, vtable). If you were relying on this in unsafe transmutes, you might get some very weird and apparently unrelated errors. You should not be doing this! Prefer not to do this at all, but if you must, you should use raw::TraitObject rather than hardcoding rustc's internal representation into your code.

2. The minimal type of reference-to-vec-literals (e.g., `&[1, 2, 3]`) is now a fixed size vec (e.g., `&[int, ..3]`) where it used to be an unsized vec (e.g., `&[int]`). If you want the unszied type, you must explicitly give the type (e.g., `let x: &[_] = &[1, 2, 3]`). Note in particular where multiple blocks must have the same type (e.g., if and else clauses, vec elements), the compiler will not coerce to the unsized type without a hint. E.g., `[&[1], &[1, 2]]` used to be a valid expression of type '[&[int]]'. It no longer type checks since the first element now has type `&[int, ..1]` and the second has type &[int, ..2]` which are incompatible.

3. The type of blocks (including functions) must be coercible to the expected type (used to be a subtype). Mostly this makes things more flexible and not less (in particular, in the case of coercing function bodies to the return type). However, in some rare cases, this is less flexible. TBH, I'm not exactly sure of the exact effects. I think the change causes us to resolve inferred type variables slightly earlier which might make us slightly more restrictive. Possibly it only affects blocks with unreachable code. E.g., `if ... { fail!(); "Hello" }` used to type check, it no longer does. The fix is to add a semicolon after the string.
Rebasing changes
Optimise a particularly clown shoes example of DST codegen
auto merge of #14397 : nick29581/rust/coerce, r=pnkfelix

DST coercions and DST fields in structs

The commits are not quite stand alone, I should probably squash them together before landing. In particular if you review the individual commits, then you'll see some scrappy stuff that gets fixed in later commits. But reading the commits in order might be easier to get an overall idea of what is going on.

The first commit includes putting back time zone into our time library - @pcwalton removed that as part of his de-~str'ing, but I had already converted it to use StrBuf, so we may as well leave it in. Update: no longer, this is removed in a later commit.