GCC (and clang) supports extensions to annotate functions so that their
results must be used and so that their arguments can't be statically
provable to be null. If a caller violates these requirements they
get a warning, so this helps them write correct code.
I deployed this in libopus a couple years ago with good success, and
the implementation here is basically copied straight from that.
One consideration is that the non-null annotation teaches the optimizer
and will actually compile out runtime non-nullness checks as dead-code.
Since this is usually not whats wanted, the non-null annotations are
disabled when compiling the library itself.
The commit also removes some dead inclusions of assert.h and introduces
compatibility macros for restrict and inline in preparation for some
portability improvements.
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.
The documentation implies that this check is happening, so make it so.
Without this check, passing an invalid nonce will trigger an internal assertion.
