secp256k1 merges @secp256k1merges@x0f.org

[secp256k1] Merged PR from real-or-random: readme: Misc improvements https://github.com/bitcoin-core/secp256k1/pull/1131

[secp256k1] Merged PR from real-or-random: configure: Remove pkgconfig macros again (reintroduced by mismerge) https://github.com/bitcoin-core/secp256k1/pull/1128

[secp256k1] Merged PR from real-or-random: config: Set preprocessor defaults for ECMULT_* config values https://github.com/bitcoin-core/secp256k1/pull/1121

[secp256k1] Merged PR from real-or-random: tests: Randomize the context with probability 15/16 instead of 1/4 https://github.com/bitcoin-core/secp256k1/pull/1122

[secp256k1] Merged PR from real-or-random: ecmult_gen: Skip RNG when creating blinding if no seed is available https://github.com/bitcoin-core/secp256k1/pull/1120

[secp256k1] Merged PR from real-or-random: build: Fix #include "..." paths to get rid of further -I arguments https://github.com/bitcoin-core/secp256k1/pull/1116

[secp256k1] Merged PR from henopied: Fix sepc256k1 -> secp256k1 typo in group.h https://github.com/bitcoin-core/secp256k1/pull/1115

[secp256k1] Merged PR from siv2r: `_scratch_destroy`: move `VERIFY_CHECK` after invalid scrach space check https://github.com/bitcoin-core/secp256k1/pull/1114

[secp256k1] Merged PR from real-or-random: ci: Add MSVC builds https://github.com/bitcoin-core/secp256k1/pull/1084

[secp256k1] Merged PR from theuni: Don't export symbols in static libraries https://github.com/bitcoin-core/secp256k1/pull/1105

[secp256k1] Merged PR from SpriteOvO: Fix the false positive of `SECP_64BIT_ASM_CHECK` https://github.com/bitcoin-core/secp256k1/pull/1104

[secp256k1] Merged PR from peterdettman: Save negations in var-time group addition https://github.com/bitcoin-core/secp256k1/pull/1056

[secp256k1] Merged PR from real-or-random: hash: Make code agnostic of endianness https://github.com/bitcoin-core/secp256k1/pull/1093

[secp256k1] Merged PR from robot-dreams: doc: Clarify configure flags for optional modules https://github.com/bitcoin-core/secp256k1/pull/1094

[secp256k1] Merged PR from real-or-random: Schnorrsig API improvements https://github.com/bitcoin-core/secp256k1/pull/1089

[secp256k1] Merged PR from real-or-random: Change SHA256 byte counter from size_t to uint64_t https://github.com/bitcoin-core/secp256k1/pull/731

[secp256k1] Merged PR from real-or-random: configure: Remove redundant pkg-config code https://github.com/bitcoin-core/secp256k1/pull/1090

[secp256k1] Merged PR from real-or-random: configure: Use modern way to set AR https://github.com/bitcoin-core/secp256k1/pull/1088

[secp256k1] Merged PR from real-or-random: configure: Add hidden --enable-dev-mode to enable all the stuff https://github.com/bitcoin-core/secp256k1/pull/1079

[secp256k1] Merged PR from elichai: Add usage examples https://github.com/bitcoin-core/secp256k1/pull/748