Tests

feat: add `abigen!` macro #890

Job	Run time
no-std build	3m 6s
Unix tests (ubuntu-latest, stable)	4m 1s
Unix tests (ubuntu-latest, nightly)	3m 30s
Unix tests (macos-11, stable)	4m 49s
Unix tests (macos-11, nightly)	5m 13s
WASM tests	9m 54s
Windows tests (stable, x86_64-pc-windows-gnu)	11m 22s
Windows tests (stable, x86_64-pc-windows-msvc)	7m 38s
Windows tests (nightly, x86_64-pc-windows-gnu)	11m 7s
Windows tests (nightly, x86_64-pc-windows-msvc)	7m 15s
	1h 7m 55s

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