Add sha256 function

[jacqueswww]

Simple Summary

Currently Vyper doesn't

Abstract

Add sha256 function, which hcalls

Motivation

Currently sha256 precompile isn't exposed as a function, this VIP adds the functionality.

Specification

def sha256(bytelike: string) -> bytes32:
      # bytelike can be one of  byte_literal, str_literal, bytes, string, bytes32
      # ...

This function will make a call to the pre-compile at address 0x000...0002, and return the output as bytes.

Backwards Compatibility

Feature, forwards compatible.

Dependencies

No dependencies.

Copyright

Copyright and related rights waived via CC0

[jacqueswww]

There is currently workaround on the deposit contract: ethereum/deposit_contract#14 using raw call, so quite keen on getting this in on b9 or b10.