Try to not leave secret data on the stack or heap.

This makes a basic effort and has not been audited.
Doesn't appear to have a measurable performance impact on bench.

It also adds a secp256k1_num_free to secp256k1_ecdsa_pubkey_create.
Make tests and bench just use asm directly instead of library

Thanks to Cory Fields for the suggestion.
Merge pull request #53

2f6c801 Try to not leave secret data on the stack or heap. (Gregory Maxwell)
Merge pull request #55

8881212 Make tests and bench just use asm directly instead of library (Pieter Wuille)
Add VERIFY_CHECK/DEBUG_CHECK and use CHECK macros more