crypto/rsa,crypto/internal/bigmod: optimized short exponentiations
RSA encryption and verification performs an exponentiation by a value
usually just a few bits long. The current strategy with table
precomputation is not efficient.
Add an ExpShort bigmod method, and use it in RSA public key operations.
After this, almost all CPU time in encryption/verification is spent
preparing the constants for the modulus, because PublicKey doesn't have
a Precompute function.
This speeds up signing a bit too, because it performs a verification to
protect against faults.