Macro hygeine of TensorCast symbol

[MilesCranmer]

Seems like there is a macro hygeine issue for complex tensor casts:

julia> using TensorCast: @cast

julia> X = ones(2, 3); Y = ones(5);

julia> @cast Z[i, j, k] := X[i, j] * Y[k];
ERROR: UndefVarError: `TensorCast` not defined
Stacktrace:
 [1] top-level scope
   @ ~/.julia/packages/TensorCast/gBujg/src/macro.jl:209

[mcabbott]

Dup of #64. Somewhere I have a branch...