Merge rust-bitcoin/rust-secp256k1#685: Release tracking PR: `v0.29.0` #688

Job	Run time
Test 32-bit version	29s
Test - beta toolchain	49s
Check for changes to the public API	1m 30s
Cross test	1m 10s
Test - 1.56.1 toolchain	5m 4s
Test - stable toolchain	3m 56s
Test - nightly toolchain	3m 41s
WASM	1m 52s
	18m 31s

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