secp256k1 merges @secp256k1merges@x0f.org

[secp256k1] Merged PR from real-or-random: gen_context: Don't use any ASM https://github.com/bitcoin-core/secp256k1/pull/965

[secp256k1] Merged PR from real-or-random: "Schnorrsig API overhaul" fixups https://github.com/bitcoin-core/secp256k1/pull/963

[secp256k1] Merged PR from jonasnick: schnorrsig API overhaul https://github.com/bitcoin-core/secp256k1/pull/844

[secp256k1] Merged PR from niooss-ledger: tests_exhaustive: check the result of secp256k1_ecdsa_sign https://github.com/bitcoin-core/secp256k1/pull/960

[secp256k1] Merged PR from UdjinM6: configure: replace AC_PATH_PROG to AC_CHECK_PROG https://github.com/bitcoin-core/secp256k1/pull/951

[secp256k1] Merged PR from real-or-random: Various improvements related to CFLAGS https://github.com/bitcoin-core/secp256k1/pull/944

[secp256k1] Merged PR from sipa: Avoid passing out-of-bound pointers to 0-size memcpy https://github.com/bitcoin-core/secp256k1/pull/879

[secp256k1] Merged PR from real-or-random: ci: Add ppc64le build https://github.com/bitcoin-core/secp256k1/pull/950

[secp256k1] Merged PR from jonasnick: Add ecmult_gen, ecmult_const and ecmult to benchmark https://github.com/bitcoin-core/secp256k1/pull/662

[secp256k1] Merged PR from real-or-random: ci: Run PRs on merge result even for i686 https://github.com/bitcoin-core/secp256k1/pull/947

[secp256k1] Merged PR from real-or-random: ci: Run ASan/LSan and reorganize sanitizer and Valgrind jobs https://github.com/bitcoin-core/secp256k1/pull/846

[secp256k1] Merged PR from real-or-random: Clean up git tree https://github.com/bitcoin-core/secp256k1/pull/941

[secp256k1] Merged PR from sipa: Add ARM32/ARM64 CI https://github.com/bitcoin-core/secp256k1/pull/930

[secp256k1] Merged PR from apoelstra: add `secp256k1_ec_pubkey_cmp` method https://github.com/bitcoin-core/secp256k1/pull/850

[secp256k1] Merged PR from real-or-random: contrib: Explain explicit header guards https://github.com/bitcoin-core/secp256k1/pull/940

[secp256k1] Merged PR from jonasnick: secp256k1.h: clarify that by default arguments must be != NULL https://github.com/bitcoin-core/secp256k1/pull/926

[secp256k1] Merged PR from roconnor-blockstream: Have ge_set_gej_var, gej_double_var and ge_set_all_gej_var initialize all fields of their outputs. https://github.com/bitcoin-core/secp256k1/pull/937

[secp256k1] Merged PR from whb07: changed include statements without prefix 'include/' https://github.com/bitcoin-core/secp256k1/pull/925

[secp256k1] Merged PR from real-or-random: Fix gen_context/ASM build on ARM https://github.com/bitcoin-core/secp256k1/pull/936

[secp256k1] Merged PR from gmaxwell: Avoids a missing brace warning in schnorrsig/tests_impl.h on old compilers https://github.com/bitcoin-core/secp256k1/pull/933