Merge rust-bitcoin/rust-bitcoin#3560: units: Remove `alloc` feature g… #109

Job	Run time
Test 32-bit version	1m 29s
Cross test - stable toolchain	1m 39s
Docs - stable toolchain (recent)	1m 20s
Kani codegen - stable toolchain	59s
Test - MSRV toolchain (minimal)	29m 12s
Prepare	6s
Test - stable toolchain (minimal)	19m 3s
Test - MSRV toolchain (recent)	30m 40s
Test - stable toolchain (recent)	19m 6s
WASM - stable toolchain	1m 30s
Bench - nightly toolchain (recent)	2m 18s
ASAN - nightly toolchain (recent)	37s
Embedded - nightly toolchain	3m 9s
Docs - nightly toolchain (recent)	42s
Lint - nightly toolchain (recent)	1m 4s
Test - nightly toolchain (minimal)	15m 0s
Test - nightly toolchain (recent)	15m 16s
	2h 23m 10s

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