Ship the Nimbus EVM as a WebAssembly package

[zah]

For the purposes of creating browser-based light clients or development environments such as Embark cockpit, we want to ship our EVM implementation as an easy-to-use WebAssembly module. Nimbus may also accidentally end up competing in the race to create the most efficient Eth1 implementation as an Eth2 execution environment.

Goals of this bounty:

• Create a Makefile target or a set of scripts that cross-compile Nimbus to WASM. We maintain similar scripts for parts of our beacon node which may prove to be helpful.

• Create a headless testing setup (e.g. based on Headless Chrome) that can execute the Eth1 test suite against the WASM implementation. Relevant files in the native test suite:

  https://github.com/status-im/nimbus/blob/master/tests/test_blockchain_json.nim
  https://github.com/status-im/nimbus/blob/master/tests/test_generalstate_json.nim
  https://github.com/status-im/nimbus/blob/master/tests/test_vm_json.nim
  https://github.com/status-im/nimbus/blob/master/tests/test_transaction_json.nim