Vyper upgrades, etc

[djrtwo]

Addresses #153, #138

What I did

• upgrade vyper to recent commit
  • custom units
  • change int128 to uint256 for all non-negative integers
  • no vyper support for convert (uint256 -> decimal) yet so there are a lot of (uint256 -> int128 -> decimal) converts for the time being
• deploy contract via irregular state change in testing
  • contract size exceeds EIP170 limits
  • good sample code for future py-evm implementation
• vyper currently drops units when doing a convert
  • add constants ONE_WEI_UINT256 and ONE_WEI_DECIMAL to be able to add wei units back in after a convert strips them.
  • This is a bug filed here Convert units should carry vyperlang/vyper#865. Looks like it will be fixed shortly
• Tests were about 10x slower when we used eth-tester==0.1.0b26 && py-evm==0.2.0a18. Downgraded to eth-tester==0.1.0b24 && py-evm==0.2.0a16 and performance went back to normal.
• upgrade web3.py to 4.3.0

[djrtwo]

A couple of build errors. Fixing now

[djrtwo]

fixed. ready for review

[ChihChengLiang]

Notice that wei_value and uint256(wei) are using interchangeably in the codebase. wei_value looks prettier while uint256(wei) provides more detail when tracing the convert.
The rest looks great!

[djrtwo]

You're right. I'm switching everything to wei_value for now.