Use bounded exponentiation to improve encrypt/verify performance

[lubux]

Limit exponentiation to the bit length of e to reduce computation time.
This is safe since e is public.

Improves encrypt/verify performance by over 2x on my laptop.

rsa_2048_pkcsv1_encrypt time:   [17.822 µs 17.849 µs 17.877 µs]
                        change: [-80.899% -80.786% -80.697%] (p = 0.00 < 0.05)
                        Performance has improved.
rsa_4096_pkcsv1_encrypt time:   [149.74 µs 150.04 µs 150.37 µs]
                        change: [-63.132% -61.496% -60.140%] (p = 0.00 < 0.05)
                        Performance has improved.
                        
                        
aws_lc_rs_2048_pkcs1_encrypt
                        time:   [10.682 µs 10.714 µs 10.747 µs]
aws_lc_rs_4096_pkcs1_encrypt
                        time:   [31.771 µs 31.826 µs 31.879 µs]