Add basic support for exceptions #290

Job	Run time
Typecheck with Agda with nix (base-lib)	1m 3s
Typecheck with Agda with nix (containers-lib)	1m 12s
Compile agda2hs with nix (agda2hs)	59s
Test building inside a nix shell	1m 26s
	4m 40s

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