Merge rust-bitcoin/rust-secp256k1#685: Release tracking PR: `v0.29.0` #1284

Job	Run time
Test - beta toolchain	31s
Test - 1.56.1 toolchain	5m 5s
Test 32-bit version	46s
Test - nightly toolchain	3m 39s
Test - stable toolchain	3m 38s
WASM	2m 1s
Cross test	1m 13s
Check for changes to the public API	1m 31s
	18m 24s

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