Stack usage in sliding-window exponentiation

[d-otte]

The sliding window for the sliding-window exponentiation is allocated (on the stack) with:

W[ 2 << MBEDTLS_MPI_WINDOW_SIZE ]

this should (most likely) be:

W[ 1 << MBEDTLS_MPI_WINDOW_SIZE ]

I ran the tests with this modification and they passed. To ensure the tests would catch actually fail if the array would be to small, I also ran them with a W[ (1 << MBEDTLS_MPI_WINDOW_SIZE) - 1], and they failed as expected.

This change will also reduce stack usage (during this function), by nearly a factor of 2! 🤩

The PR #3592 contains the modification.