Updating secp256k1 and linking with external callbacks

[elichai]

This is the first step of #114

• Updated secp256k1 library.
• Added ECMULT_WINDOW_SIZE precompile flag with the default 15 (source at the bottom)
• Implemented a the callbacks in rust to be linked via USE_EXTERNAL_DEFAULT_CALLBACKS

After this PR all no-std environments that supports liballoc should work (because they should have access to malloc)
But my next step is to use the pre-allocated feature.

Refs:
https://github.com/bitcoin-core/secp256k1/blob/master/configure.ac#L422
https://github.com/bitcoin-core/secp256k1/blob/master/configure.ac#L406

[elichai]

One thing I'm not sure of is if we should do anything with the message in the callback, because doing anything with it will require libc for no-std (for strlen)

[elichai]

I implemented strlen so we can print the message.
if some version of bitcoin-core/secp256k1#639 will get merged upstream I'll remove the implementation and change the callbacks accordingly.

[elichai]

Rebased, replaced the c_char with i8 and a warning.
and removed some comment I missed about derefing the callback message.

[apoelstra]

Confirmed this eliminates fprintf and abort from the missing symbols in rustwasm/wasm-pack#621

Only remaining ones are malloc and free. ACK.