Add slice and gather syntax

Summary:
Implemented syntactic sugar for the following constructs:

- `x.Gather(y)` can now be written as `x[y]`
- `x.Slice(start, end)` can now be written as `x[start:end]`

For slicing, `start` and/or `end` can be omitted iff `x` is one-dimensional (i.e. a vector). That is, `vector[start:]`, `vector[:end]` and `vector[:]` will work. Doesn't work for higher-dimensional tensors because to emit the start/end indices we need to know the rank of the tensor (since `Slice` requires one entry per dimension of the tensor).

Also added a `getProto()` function so that I could test that the generated code is as expected (i.e. that the syntactic sugar does not affect the structure of the output).

Reviewed By: zdevito

Differential Revision: D6605864

fbshipit-source-id: 786359713a13314c24be2fc07e01486c507404ef