seth calldata through hevm encode (#374)

* hevm-cli.hs: expose abi encoding/decoding

* hevm: expose hevm abiencode cli

* seth: proof of concept new calldata style

* add some basic seth calldata tests

* use hevm for seth calldata

* hevm: better abi value parsing

* make hevm a dependency of seth

* fix bool test

* skip staticcall50000

* fix shellcheck stuff

* cleanup

* changelog entries for seth and hevm

* bump seth version

* skip memory intense Return50000 test

* src/seth/default.nix: add hevm to PATH in wrapProgram

* fix signature extraction

* ensure test script errors on failure

* dapp install ds-test

* fix constantinople tests

* fix missing quotes

* remove set -x in `seth calldata`

* compare strings with `=` instead of `-eq`

Co-authored-by: Lev Livnev <lev@liv.nev.org.uk>

dapp: 0.26.0 -> 0.27.0

seth: 0.8.2 -> 0.8.4

We had missed this while releasing seth in the previous commits.

Merge pull request #314 from dapphub/solc-0.5.15

solc 0.5.15

dapp: 0.25.0

hevm 0.34

seth: 0.8.3

hevm 0.33

dapp: update version

Merge pull request #216 from dapphub/dapp-build-fast

dapp: faster builds with single solc pass