Clear sensitive memory without getting optimized out (revival of #636)

[theStack]

This PR picks up #636 (which in turn picked up #448, so this is take number three) and is essentially a rebase on master.

Some changes to the original PR:

• the clearing function now has the secp256k1_ prefix again, since the related helper _memczero got it as well (see PR #835 / commit e89278f211a526062745c391d48a7baf782b4b2b)
• the original commit b17a7df8145a6a86d49c354c7e7b59a432ea5346 ("Make _set_fe_int( . , 0 ) set magnitude to 0") is not needed anymore, since it was already applied in PR #943 (commit d49011f54c2b31807158bdf06364f331558cccc7)
• clearing of stack memory with secp256k1_memclear is now also done on modules that have been newly introduced since then, i.e. schnorr and ellswift (of course, there is still no guarantee that all places where clearing is necessary are covered)

So far I haven't looked at any disassembly and possible performance implications yet (there were some concerns expressed in https://github.com/bitcoin-core/secp256k1/pull/636#issuecomment-620118629), happy to go deeper there if this gets Concept ACKed.

The proposed method of using a memory barrier to prevent optimizating away the memset is still used in BoringSSL (where it was originally picked up from) and in the Linux Kernel, see e.g. https://github.com/google/boringssl/blob/5af122c3dfc163b5d1859f1f450756e8e320a142/crypto/mem.c#L335 and https://github.com/torvalds/linux/blob/d4560686726f7a357922f300fc81f5964be8df04/include/linux/string.h#L348 / https://github.com/torvalds/linux/blob/d4560686726f7a357922f300fc81f5964be8df04/include/linux/compiler.h#L102

Fixes #185.

[theStack]

We should call SECP256K1_CHECKMEM_UNDEFINE (

https://github.com/bitcoin-core/secp256k1/blob/b307614401790850b48fb3ba878247290857a975/src/checkmem.h#L27

) in secp256k1_memclear after clearing the memory. Reading cleared memory would clearly be a bug.

Thanks, added that, and rebased on master.