Edits and checks for modulus, division, and exponentiation

[iamdefinitelyahuman]

What I did

• Modify modulus calculations for literals to be consistent with the EVM - closes #1693
• Explicitly disallow the use of exponentiation on decimal values - closes #1698
• Add compile-time checks for divide by zero and modulo by zero

How I did it

• For literal modulus calculations, calculate abs(a) % abs(b) and then return a negative value if a < 0. This is consistent with the results of SMOD.
• Add ZeroDivisionException to allow raising zero division errors with code highlights
• various minor edits to vyper/parser/expr.py - mostly raising exceptions

How to verify it

Run the tests. I added more test cases around these behaviors.

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[fubuloubu]

Please rebase tho