Improve clarity of reserve types and trading function invariant

[Alexangelj]

Reserves can be in the following format:

• Reserves in decimal
• Reserves scaled from decimal to wad
• Reserves in wad per liquidity

Trading function invariant could accept the following reserves:

• Reserves in wad per liquidity, rounded up
• Reserves in wad per liquidity, rounded down
• Whatever combination of reserves of the above types

This makes it increasingly more confusing and difficult to replicate the exact values in non-solidity environments, and also in solidity tests.