The semantics of type variable matching isn't precisely specified

In our docs, we've referred to a datashape like `A * A * int32` as a square matrix, but if `A` is permitted to match the `var` dimension, this is actually a ragged array.

Since `3 * int32` is syntactic sugar for `fixed[3] * int32`, we can specify a square matrix as the more wordy `fixed[N] * fixed[N] * int32`. (Note that the pattern matching implementation does not match type variables inside of type constructors presently though.).