Python-like enumerate()

Python-like enumerate()

Summary:
Range-based for cannot be used if the element index is needed along
with the element. In these situations, it is often necessary to fall
back to the standard for loop, which is easy to get wrong, or maintain
an extra count variable, which is error-prone when control flow is
nontrivial (for example in the presence of `continue`).

This diff introduces a simple implementation of Python's
`enumerate()`, with the same signature. Since in C++ tuple-unpacking
is verbose, the iteration variable returned is a proxy object `it`
where the iteration index can be retrieved with `it.idx`, and the
value with `*it` or `it->...`, like a normal iterator.

Differential Revision: D3477877

fbshipit-source-id: 376af7f559e8b60f02a3f81f0c026a901e23ddcf