Handle BN_CTX at the EC API boundary.

Handle BN_CTX at the EC API boundary.

The EC API allows callers to optionally pass in a BN_CTX, which means that
any code needing a BN_CTX has to check if one was provided, allocate one if
not, then free it again. Rather than doing this dance throughout the EC
code, handle the BN_CTX existance at the EC API boundary. This means that
lower level implementation code can simply assume that the BN_CTX is
available.

ok tb@