Put assertions and post-conditions in the generic transactions

[joshuahannan]

Issue To Be Solved

If we are going to encourage projects to use the generic token transactions, we should add assertions and/or post-conditions to them to make sure that they are behaving properly.

Suggest A Solution

Add these conditions:

• Make sure the withdrawn tokens are the same type that is defined by the specified contract
• Make sure it has the correct balance
• Think of others