Trade-Finance Complex (replaces "Market")

Borrowing the concept from that of the "military industrial complex" building - this building incorporates the various parts of the non-governmental trade and finance sectors into one. It will use the "market" building from the game, and will probably be named as "market" but it will not just represent the usual city market square.

--- LEVEL 1 TFC ---
Cost: 1 turn, 800 mnai
+trade_income_bonus
+5% happiness

This represents the decision by the local government to allot a plot of land for local trading purposes. At this level, corruption because of this is essentially nonexistent, and happiness is improved, but the trade bonus is minimal.

--- LEVEL 2 TFC ---
Cost: 4 turns, 1600 mnai
+trade_income_bonus
+10% happiness

This represents the decision by the local government to allot greater amounts of land to the public place in which the market is located, and do a minimal amount of construction for its benefit. Little significant commerce is taking place, but it's an improvement and the people are happy.

--- LEVEL 3 TFC ---
Cost: 4 turns, 3200 mnai
+trade_level_bonus
-1% tax income
+15% happiness
-5% law (corruption)

This represents the decision by the local government to allot an even greater amount of land to the public place for trade, and to spend money for things such as public weights tables, statues, and police to ensure things are carried out fairly. Some amount of corruption goes on involving government officials and finance, but it's limited.

--- LEVEL 4 TFC ---
Cost: 4 turns, 3200 mnai
+trade_level_bonus
-2% tax income
+20% happiness
-10% law (corruption)

This represents a further expansion of government support for the TFC. Happiness and corruption have increased in equal measure, tax costs for support went up, and the trade level bonus increased, all in the same amount.

--- LEVEL 5 TFC ---
Cost: 4 turns, 3200 mnai
+trade_level_bonus
-3% tax income
+25% happiness
-15% law (corruption)

This represents an even further expansion of government support for the TFC. Happiness and corruption have increased in equal measure, tax costs for support went up, and the trade level bonus increased, all in the same amount. This is the highest level attainable.

I suggest using 1 turn instead of 0 turn buildings, 0 turn buildings can't be built if construction time bonus present via other buildings or traits (EB didn't have that, mind you)

That sounds reasonable.