Tests

feat: add `abigen!` macro #988

Job	Run time
Unix tests (ubuntu-latest, stable)	12m 23s
no-std build	1m 26s
Unix tests (ubuntu-latest, nightly)	12m 24s
Unix tests (macos-11, stable)	12m 26s
WASM tests	8m 6s
Unix tests (macos-11, nightly)	12m 26s
Windows tests (stable, x86_64-pc-windows-gnu)	15m 2s
Windows tests (stable, x86_64-pc-windows-msvc)	14m 52s
Windows tests (nightly, x86_64-pc-windows-gnu)	14m 51s
Windows tests (nightly, x86_64-pc-windows-msvc)	14m 46s
	1h 58m 42s

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