Merge rust-bitcoin/rust-secp256k1#685: Release tracking PR: `v0.29.0` #689

Job	Run time
Test - beta toolchain	28s
Test 32-bit version	1m 6s
Check for changes to the public API	1m 25s
Cross test	1m 4s
Test - 1.56.1 toolchain	5m 56s
Test - nightly toolchain	3m 38s
Test - stable toolchain	3m 52s
WASM	1m 46s
	19m 15s

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