Add Cartesian Embedding methods

[ldeso]

This is an alternative to #668 that adds Cartesian indexing methods to the existing Embedding layer instead of creating a new CartesianEmbedding layer.

Note that this modifies the Embedding composite type from:

struct Embedding <: AbstractExplicitLayer
    in_dims::Int
    out_dims::Int
    init_weight
end

to:

struct Embedding{N} <: AbstractExplicitLayer
    in_dims::NTuple{N, Int}
    out_dims::Int
    init_weight
end

Closes #668.

[ldeso]

Force-pushed because I forgot to capitalize the word "Cartesian" in the documentation.