Address Jens' review comments (PR#35)

Changes:
- Remove bignum typedef, use 'struct bignum'
- Bignum allocation function (in crypto_ops) now takes a size argument
- Move bignum allocation functions (for key structs) to crypto_ops;
  discard the now unused 'preallocate' function
- Suppress uppercase/camel case: N, dP, dQ, qP => n, dp, dq, qp
- Delete white lines in switch statements
- Move tee_cryp_init to init.c