Rename Pod into Copy

Summary:
So far, we've used the term POD "Plain Old Data" to refer to types that
can be safely copied. However, this term is not consistent with the
other built-in bounds that use verbs instead. This patch renames the Pod
kind into Copy.

RFC: 0003-opt-in-builtin-traits

Test Plan: make check

Reviewers: cmr

Differential Revision: http://phabricator.octayn.net/D3
auto merge of #13160 : FlaPer87/rust/rename-pod, r=thestinger

So far, we've used the term POD "Plain Old Data" to refer to types that
can be safely copied. However, this term is not consistent with the
other built-in bounds that use verbs instead. This patch renames the `Pod`
kind into `Copy`.

RFC: 0003-opt-in-builtin-traits

r? @nikomatsakis