Add Toeplitz(A::AbstractMatrix) constructor that projects onto a Toeplitz matrix

[dlfivefifty]

In Base, it's always the convention that MyMatrix(A) projects A onto the appropriate space: for example, we have Diagonal(A) which ignores off-diagonal entries, and similarly, UpperTriangular(A) and Symmetric(A).

I propose Toeplitz(A::AbstractMatrix) takes the first row and column of A and creates a Toeplitz matrix. This has the benefit that it would automatically support FillArrays.jl syntax Toeplitz(Zeros(n,m)), Toeplitz(Fill(x,n,m)) and Toeplitz(Eye(n)) for creating zero, constant and identity Toeplitz matrices, without needing to depend on FillArrays.jl.

Note that convert(Toeplitz, A) should throw an error if A is not already Toeplitz.