Tests

feat: add `abigen!` macro #916

Job	Run time
no-std build	3m 18s
Unix tests (ubuntu-latest, stable)	3m 45s
WASM tests	7m 34s
Unix tests (ubuntu-latest, nightly)	4m 17s
Windows tests (stable, x86_64-pc-windows-gnu)	10m 19s
Unix tests (macos-11, stable)	6m 33s
Windows tests (stable, x86_64-pc-windows-msvc)	9m 48s
Unix tests (macos-11, nightly)	6m 32s
Windows tests (nightly, x86_64-pc-windows-gnu)	8m 59s
Windows tests (nightly, x86_64-pc-windows-msvc)	7m 48s
	1h 8m 53s

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