Add support for doing RSA private key operations for keys with public modulus up to 16384 bits

[briansmith]

Currently, ring limits the size of the public modulus of RSA private key operations to 4096 bits. This is in line with NIST/FIPS guidance that suggested (at the time, at least) using only 3072 or 4096 bit keys.

The underlying bigint math functions use stack space roughly in proportion of a multiple of the actual size of the modulus (the assembly-language squaring and multiplication) or the maximum allowed size, depending on the function. Thus, using too much stack space and/or triggering stack overflow is a very real issue when key sizes are 16384 bits (2048 bytes).

When the key is that large, the operation doesn't need to be particularly fast; we just need to make sure it works and is safe to use. Thus, we might use a different implementation strategy for these large keys that trades speed for less space usage.