Add anvil RPC node support

[Psirex]

Currently, only acceptance tests might be run with the anvil node. How to run tests:

1. Spawn anvil node in one tab: anvil --fork-url=https://mainnet.infura.io/v3/YOUR_INFURA_KEY --base-fee 0 --auto-impersonate --steps-tracing --mnemonic "test test test test test test test test test test test junk"
2. Run acceptance tests in another tab: brownie test ./tests/acceptance --network mainnet-fork

The question is how faster is the anvil node over the ganache. My results are very strange:

• anvil: 45.38s user 2.55s system 8% cpu 9:37.67 total
• ganache: 35.79s user 1.96s system 20% cpu 3:08.60 total

The ganache node is almost 3 times faster.