librustc: Implement a `Pod` kind for types that can be `memcpy`'d.

This will be used for the new `Cell`.
libstd: Implement the new `Cell`.
test: Add a test that POD types can be implicitly copied.
doc: Update the documentation to describe the `Pod` trait.
auto merge of #10924 : pcwalton/rust/pod, r=pcwalton

This will be used for the new `Cell`.

r? @nikomatsakis